Heat transfer device

ABSTRACT

A heat transfer assembly comprising: an elongate envelope; an elongate heat transfer device located within the envelope, the heat transfer device having an elongate heat transfer chamber; and an elongate heat exchanger passing longitudinally through at least a portion of the elongate heat transfer chamber.

FIELD OF THE INVENTION

This invention relates to heat transfer devices and in particular heattransfer devices for use in solar energy converter devices which convertincident solar energy into heat and electricity.

Devices converting solar energy into electricity are known. One means ofconverting solar energy into electricity is the use of photovoltaicarrays. Photovoltaic arrays generally consist of semi-conductormaterials appropriately encapsulated, and arranged to generateelectricity when exposed to solar radiation.

Separately, devices converting solar energy into useable heat are known.A variety of thermal collection devices are known which absorb heatenergy when exposed to solar radiation. These thermal solar collectorsheat up as they absorb heat energy from solar radiation and this heatenergy may then be extracted for use, for example by pumping a liquidflow, such as water, through the thermal collector in order to heat theliquid.

It has been proposed to combine these two technologies to provide ahybrid solar energy collector converting solar energy simultaneouslyinto both electricity and heat. Such hybrid devices have been found tosuffer from the problem that the elements of the photovoltaic arraybecome hot when the device is operating. In general, the efficiency ofphotovoltaic elements drops as their temperature increases. Also, ingeneral, photovoltaic elements subject to high temperatures may sufferdegradation resulting in a permanent decrease in efficiency. As aresult, in use, the electricity generating efficiency of thephotovoltaic arrays of such hybrid devices tends to be low, and tends toreduce over time.

Accordingly, a heat transfer device suitable to transfer heat away froma solar collector is desirable.

SUMMARY OF THE INVENTION

A first aspect provides a heat transfer assembly comprising:

an elongate envelope;an elongate heat transfer device located within the envelope, the heattransfer device having an elongate heat transfer chamber; andan elongate heat exchanger passing longitudinally through at least aportion of the elongate heat transfer chamber.

Preferably, the elongate envelope is transparent or translucent.

Preferably, the elongate envelope is glass.

Preferably, the elongate heat transfer chamber extends alongsubstantially the whole length of the elongate heat transfer device.

Preferably, the elongate heat exchanger extends along substantially thewhole length of the elongate heat transfer chamber.

Preferably, the elongate heat exchanger comprises a tube which passesthrough a first end of the elongate heat transfer chamber, extends alongsubstantially the whole length of the elongate heat transfer chamber,and turns back to pass for a second time through the first end of theelongate heat transfer chamber.

Preferably, the elongate heat exchanger comprises a tube which passestwice through a first end of the elongate envelope.

Preferably, the elongate heat exchanger comprises a tube which passesthrough a first end of the elongate heat transfer chamber, extends alongsubstantially the whole length of the elongate heat transfer chamber,and passes through a second end of the elongate heat transfer chamberopposite the first end.

Preferably, the tube and/or the elongate heat transfer device comprisemeans to accommodate differential thermal expansion between the tube andelongate heat transfer device.

Preferably, the means to accommodate differential thermal expansioncomprise a bellows structure of the tube.

Preferably, the elongate heat exchanger comprises a tube which passesthrough a first end of the elongate envelope, extends alongsubstantially the whole length of the elongate envelope, and passesthrough a second end of the elongate envelope opposite the first end.

Preferably, the tube and/or the elongate envelope comprise means toaccommodate differential thermal expansion between the tube and elongateheat transfer device, and the elongate envelope.

Preferably, the means to accommodate differential thermal expansioncomprise bends in the tube.

Preferably, the elongate heat exchanger comprises a tube which passesthrough a first end of the elongate envelope, passes through theelongate heat transfer chamber, and turns back to pass for a second timethrough the first end of the elongate envelope.

Preferably, the elongate heat exchanger comprises inner and outerconcentric tubes which pass through a first end of the elongate heattransfer chamber and extend along substantially the whole length of theelongate heat transfer chamber, wherein the outer concentric tube isclosed at an end remote from the first end of the elongate heat transferchamber.

Preferably, the elongate envelope is at least partially evacuated.

Preferably, the heat transfer assembly further comprises at least onephotovoltaic element mounted on the elongate heat transfer device.

Preferably, the elongate heat transfer chamber is a vapor chamber.

Preferably, the vapor chamber is at least partially evacuated.

Preferably, the heat transfer assembly is arranged for rotation about arotation axis.

Preferably, the elongate envelope is cylindrical and has an axis ofsymmetry, and the axis of rotation is parallel to the axis of symmetry.

Preferably, the axis of rotation and the axis of symmetry are coaxial.

Preferably, the axis of rotation coincides with the location of a tubepassing through an end of the elongate envelope.

Preferably, the tube passes twice through the first end of the elongateenvelope and the axis of rotation passes between the locations at whichthe tubes passing through the end of the elongate envelope.

Preferably, the axis of rotation passes centrally between the locationsat which the tubes passing through the end of the elongate envelope.

A second aspect provides a solar collector array comprising a pluralityof heat transfer assemblies according to the first aspect mounted inparallel on a common supporting structure.

Preferably, the solar collector array further comprises means forsynchronously rotating all of the plurality of heat transfer assembliesrelative to the supporting structure about their respective axes ofrotation.

Preferably, the solar collector array further comprises means forrotating the supporting structure about an axis perpendicular to theaxes of rotation of the heat transfer assemblies.

A third aspect provides a heat transfer device comprising:

a plurality of fluid flow chambers;a common heat transfer chamber serving all of the fluid flow chambers;a heat exchanger passing through at least a portion of the heat transferchamber.

Preferably, the heat transfer device comprises more than two fluid flowchambers.

A third aspect provides a heat transfer device comprising:

a working fluid pathway;a heat transfer chamber located on the working fluid pathway;a heat exchanger passing through at least a portion of the heat transferchamber.

Preferably, the heat transfer device comprises a plurality of workingfluid pathways.

Preferably, the heat transfer chamber extends along substantially thewhole length of the heat transfer device.

Preferably, the heat exchanger extends along substantially the wholelength of the heat transfer chamber.

Preferably, the heat exchanger comprises a tube which passes through afirst end of the heat transfer chamber, extends along substantially thewhole length of the heat transfer chamber, and turns back to pass for asecond time through the first end of the heat transfer chamber.

Preferably, the heat exchanger comprises a tube which passes through afirst end of the heat transfer chamber, extends along substantially thewhole length of the heat transfer chamber, and passes through a secondend of the heat transfer chamber opposite the first end.

Preferably, the tube and/or the heat transfer device comprise means toaccommodate differential thermal expansion between the tube and heattransfer device.

Preferably, the means to accommodate differential thermal expansioncomprise a bellows structure of the tube.

Preferably, the heat exchanger comprises inner and outer concentrictubes which pass through a first end of the heat transfer chamber andextend along substantially the whole length of the heat transferchamber, wherein the outer concentric tube is closed at an end remotefrom the first end of the heat transfer chamber.

Preferably, the heat transfer device further comprises at least onephotovoltaic element mounted on the heat transfer device.

Preferably, the heat transfer chamber is a vapor chamber.

Preferably, the vapor chamber is at least partially evacuated.

Preferably, the heat transfer assembly further comprises an envelope,the heat transfer device being located within the envelope.

Preferably, the envelope is transparent or translucent.

Preferably, the envelope is glass.

Preferably, the heat exchanger comprises a tube which passes through afirst end of the heat transfer chamber, extends along substantially thewhole length of the heat transfer chamber, and turns back to pass for asecond time through the first end of the heat transfer chamber, andfurther comprises a tube which passes twice through a first end of theenvelope.

Preferably, the heat exchanger comprises a tube which passes through afirst end of the heat transfer chamber, extends along substantially thewhole length of the heat transfer chamber, and passes through a secondend of the heat transfer chamber opposite the first end, and furthercomprises a tube which passes through a first end of the envelope,extends along substantially the whole length of the envelope, and passesthrough a second end of the envelope opposite the first end.

Preferably, the tube and/or the envelope comprise means to accommodatedifferential thermal expansion between the tube and heat transferdevice, and the envelope.

Preferably, the means to accommodate differential thermal expansioncomprise bends in the tube.

Preferably, the heat exchanger comprises a tube which passes through afirst end of the heat transfer chamber, extends along substantially thewhole length of the heat transfer chamber, and passes through a secondend of the heat transfer chamber opposite the first end, and furthercomprises a tube which passes through a first end of the envelope,passes through the heat transfer chamber, and turns back to pass for asecond time through the first end of the envelope.

Preferably, the heat transfer assembly is arranged for rotation about arotation axis.

Preferably, the elongate envelope is cylindrical and has an axis ofsymmetry, and the axis of rotation is parallel to the axis of symmetry.

Preferably, the axis of rotation and the axis of symmetry are coaxial.

Preferably, the axis of rotation coincides with the location of a tubepassing through an end of the envelope.

Preferably, the tube passes twice through the first end of the envelopeand the axis of rotation passes between the locations at which the tubespass through the first end of the envelope.

Preferably, the axis of rotation passes centrally between the locationsat which the tubes pass through the first end of the elongate envelope.

A fourth aspect provides a solar collector array comprising a pluralityof heat transfer assemblies according to any the second or third aspectsmounted in parallel on a common supporting structure.

Preferably, the solar collector array further comprises means forsynchronously rotating all of the plurality of heat transfer assembliesrelative to the supporting structure about their respective axes ofrotation.

Preferably, the solar collector array further comprises means forrotating the supporting structure about an axis perpendicular to theaxes of rotation of the heat transfer assemblies.

A fifth aspect provides a heat transfer assembly comprising a pluralityof connected heat transfer devices each according to the second or thirdaspects, wherein each heat transfer device has a separate heat transferchamber; and

an envelope;the plurality of heat transfer devices being located within theenvelope.

Preferably, the heat transfer assembly further comprises a heat exchangenetwork, the heat exchange network connecting the respective heatexchangers of the plurality of heat transfer devices.

Preferably, the envelope is an elongate envelope.

Preferably, the envelope is transparent or translucent.

Preferably, the envelope is glass.

Preferably, the heat exchange network comprises a plurality of tubeswhich pass through an end of the envelope.

Preferably, the envelope is at least partially evacuated.

Preferably, the heat transfer assembly is arranged for rotation about arotation axis.

Preferably, the envelope is cylindrical and has an axis of symmetry, andthe axis of rotation is parallel to the axis of symmetry.

Preferably, the axis of rotation and the axis of symmetry are coaxial.

Preferably, the axis of rotation coincides with the location of a tubepassing through an end of the envelope.

A sixth aspect provides a solar collector array comprising a pluralityof heat transfer assemblies according to the fifth aspect mounted inparallel on a common supporting structure.

Preferably, the solar collector array further comprises means forsynchronously rotating all of the plurality of heat transfer assembliesrelative to the supporting structure about their respective axes ofrotation.

Preferably, the solar collector array further comprises means forrotating the supporting structure about an axis perpendicular to theaxes of rotation of the heat transfer assemblies.

A seventh aspect provides a heat transfer device comprising:

a fluid flow means partially filled with a liquid and arranged so that afirst surface is in thermal contact with the liquid in a part of thefluid flow means inclined to the horizontal and containing the liquid;

-   -   the first part of the fluid flow means being divided into a        plurality of first fluid flow channels each having an upper end        and a lower end and at least one second fluid flow channel        having an upper end and a lower end arranged so that the liquid        in the first fluid flow channels is in better thermal contact        with the first surface than the liquid in the second fluid flow        channel; and    -   the upper ends of the first and second fluid flow channels being        connected together by a vapor manifold, and a second surface        being located in the vapor manifold;    -   wherein the vapor manifold is at least partially evacuated;    -   whereby, when the first surface is hotter than the second        surface, heat energy from the first surface causes the liquid in        the first fluid flow channel to vaporize, and the vapor travels        through the liquid in the first fluid flow channel to the        surface of the liquid, such that the liquid circulates around        the first fluid flow channel and the second fluid flow channel;    -   vapor travels from the surface of the liquid through the vapor        manifold to the second surface and condenses at the second        surface; and    -   condensed liquid returns from the second surface to the first        part of the fluid flow means;    -   whereby heat energy is transported from the first surface to the        second surface.

Preferably, the second surface is a surface of a heat exchangercontaining a fluid, whereby, when the first surface is hotter than thefluid, heat energy is transported from the first surface to the fluid.

Preferably, the second surface is an external surface of a tubecontaining the fluid.

Preferably, the fluid is arranged to flow through the tube.

Preferably, the vapor manifold extends between first and second opposedsurfaces, and the tube passes through the first surface, extends throughthe vapor manifold between the first and second surfaces, and passesthrough the second surface.

Preferably, the vapor manifold comprises a surface, and the tube passesthrough the surface, extends within the vapor manifold, and passesthrough the surface for a second time.

Preferably, the vapor manifold comprises a surface, the tube passesthrough the first surface and extends within the vapor manifold; and

an inner tube is arranged within the tube, whereby the fluid is arrangedto flow through the inner tube, and to flow between the tube and theinner tube.

Preferably, the first part of the fluid flow means is divided into aplurality of first fluid flow channels and a plurality of second fluidflow channels.

Preferably, the number of first fluid flow channels is the same as thenumber of second fluid flow channels.

Preferably, the first and second fluid flow channels are located side byside with first fluid flow channels and second fluid flow channelsinterleaved.

Preferably, the cross sectional area of the first fluid flow channel andthe cross sectional area of the second fluid flow channel are equal.

Preferably, the first fluid flow channel is in thermal contact with thefirst surface across a greater area than the second fluid flow channel.

Preferably, the lower ends of the first and second fluid flow channelsare connected together.

Preferably, the first part of the fluid flow means is inclined to thehorizontal by an angle of up to 90°.

Preferably, the liquid comprises water.

Preferably, the liquid comprises ethanol.

Preferably, the liquid comprises a mixture of water and ethanol.

Preferably, the mixture comprises up to 25% ethanol.

Preferably, the fluid comprises water.

Preferably, the part of the fluid flow means above the surface of theliquid is at a pressure of 40 mbar or less.

Preferably, the part of the fluid flow means above the surface of theliquid is at a pressure of 2 mbar or less.

Preferably, the part of the fluid flow means above the surface of theliquid is at a pressure of 1 mbar or less.

Preferably, the part of the fluid flow means above the surface of theliquid is at a pressure of 10⁻² mbar or less.

Preferably, the part of the fluid flow means above the surface of theliquid is at a pressure of 10⁻³ mbar or less.

Preferably, the part of the fluid flow means above the surface of theliquid is at a pressure of 10⁻⁶ mbar or less.

Preferably, the first fluid flow channels are closer to the firstsurface than the second fluid flow channels.

Preferably, at least a part of each first fluid flow channel is locatedbetween the first surface and a second fluid flow channel.

Preferably, the first fluid flow channels lie between the first surfaceand the second fluid flow channels.

Preferably, each of the first and second fluid flow channels has asection bounded by a perimeter, and a proportion of the perimeter of thefirst fluid flow channel which is in thermal contact with the firstsurface is greater than a proportion of the perimeter of the secondfluid flow channel which is in thermal contact with the first surface.

Preferably, at least a portion of at least one surface of each firstfluid flow channel in thermal contact with the first surface comprisesfeatures arranged to promote vapor bubble nucleation.

Preferably, at least a portion of at least one surface of each firstfluid flow channel in thermal contact with the first surface has asurface texture adapted to promote vapor bubble nucleation.

Preferably, said portion of at least one surface has a roughened surfacetexture.

Preferably, the roughened surface texture is provided by a solder layer.

Preferably, vapor traveling from the surface of the liquid to the secondsurface passes through the manifold.

Preferably, condensed liquid returning from the second surface to thefirst part of the fluid flow means passes through the manifold.

Preferably, the second surface is located above the first surface suchthat the condensed liquid returns from the second surface to the firstpart of the fluid flow means by gravity.

Preferably, at least a portion of a surface of each first fluid flowchannel in thermal contact with the first surface has a dimpled surfaceprofile.

Preferably, the dimpled surface profile comprises a regular array ofdimples.

Preferably, the regular array of dimples comprises dimples arranged inrows separated by flat strips without dimples.

Preferably, the first and second fluid flow channels are located betweenfirst and second spaced apart plates.

Preferably, the first plate is in thermal contact with the first surfaceand forms a surface of the or each first fluid flow channel.

Preferably, there are a plurality of first fluid flow channels and aplurality of second fluid flow channels located side by side with firstfluid flow channels and second fluid flow channels arranged alternately,and each first fluid flow channel is separated from an adjacent secondfluid flow channel by a partition extending between and attached to thefirst plate and the second plate.

Preferably, the first plate has a dimpled surface profile comprising aregular array of dimples arranged in rows separated by flat stripswithout dimples, and each partition is attached to the first plate at aposition located in one of the flat strips.

Preferably, the part of each partition extending between the first plateand the second plate is substantially flat.

Preferably, a plurality of the partitions are formed by a third plate.

Preferably, all of the partitions are formed by a single third plate.

Preferably, the third plate is corrugated.

Preferably, each of the plates comprises a metal or a metal alloymaterial.

Preferably, each of the plates comprises mild steel.

Preferably, each of the plates comprises tin coated mild steel.

Preferably, the plates are bonded together by a bonding techniqueincluding at least one of: soldering; spot welding; roller welding; andan adhesive.

Preferably, the plates are bonded together by solder joints and at leasta part of the first plate forming a surface of each first fluid flowchannel is coated with solder.

Preferably, the heat transfer device comprises a substantially rigidheat conducting structure.

An eighth aspect provides a heat transfer device comprising:

a plurality of first fluid flow channels each having an upper and alower end, inclined to the horizontal, and containing a liquid;a second fluid flow channel having an upper and a lower end, connectedto the first fluid flow channels and containing the liquid;a vapor manifold connecting the upper ends of the first and second fluidflow channels;a first surface in thermal contact with the liquid in the first fluidflow channel; anda second surface in the vapor manifold;whereby when the first surface is hotter than the second surface, heatenergy from the first surface causes liquid in the first fluid flowchannels to vaporize;the vapor travels upwardly along the first fluid flow channels;the vapor drives a flow of liquid from the second fluid flow channel tothe first fluid flow channels and upwardly along the first fluid flowchannels; andthe vapor travels from a surface of the liquid to the second surface andcondenses at the second surface;whereby heat energy is transported away from the first surface to thesecond surface.

A ninth aspect provides a heat transfer device comprising:

a first surface;a second surface;a liquid reservoir in thermal contact with the first surface andcontaining a liquid;wherein the liquid reservoir comprises a plurality of first fluid flowchannels inclined to the horizontal and containing the liquid and asecond fluid flow channel connected to the first fluid flow channel andcontaining the liquid;the device further comprising a vapor manifold connecting upper ends ofthe first and second fluid flow channels;the first surface is in thermal contact with the liquid in the firstfluid flow channel, the second surface is in the vapor manifold; andthe vapor manifold is at least partially evacuated;whereby when the first surface is hotter than the second surface, heatenergy from the first surface causes liquid in the first fluid flowchannel to vaporize;the vapor travels upwardly along the first fluid flow channel, into thevapor manifold, and condenses at the second surface;the vapor drives a flow of liquid from the second fluid flow channel tothe first fluid flow channel and upwardly along the first fluid flowchannel; andcondensed liquid returns from the second surface to the liquidreservoir;whereby heat energy is transported away from the first surface to thesecond surface.

A tenth aspect provides a heat transfer device comprising:

a first surface;a second surface;a liquid reservoir in thermal contact with the first surface andcontaining a liquid; anda vapor manifold containing the second surface;wherein at least a part of the vapor manifold is at least partiallyevacuated;whereby when the first surface is hotter than the second surface, heatenergy from the first surface causes liquid in the liquid reservoir tovaporize;the vapor travels to the vapor manifold and condenses at the secondsurface; andcondensed liquid returns from the second surface to the liquidreservoir;whereby heat energy is transported from the first surface to the secondsurface.

Preferably, at least a part of the heat transfer device is located in anenvelope under at least a partial vacuum.

Preferably, the envelope is one of: a cylindrical tube; an ellipticaltube.

Preferably, the envelope is formed, at least in part, of glass.

Preferably, a plurality of tubes are mounted in a solar energycollecting array.

Preferably, at least one of the plurality of tubes is rotatable to tracklight incident on the solar energy collecting array.

Preferably, the plurality of tubes are rotatable to track light incidenton the solar energy collecting array.

Preferably, the heat transfer device comprises a substantially rigidheat conducting structure.

A sixth aspect provides an energy generator comprising a heat transferdevice according to any preceding claim, and at least one photovoltaicelement, the energy generator having an electrical output and a heatedfluid output.

The invention further provides systems, devices and articles ofmanufacture for implementing any of the aforementioned aspects of theinvention.

DESCRIPTION OF FIGURES

The invention will now be described in detail with reference to thefollowing figures in which:

FIG. 1 is a diagram of a first embodiment of a hybrid solar energyconverter according to the invention;

FIG. 2 is a diagram of a tube useable in the hybrid solar energyconverter of FIG. 1;

FIG. 3 is a cut away diagram of a solar energy collector assemblyuseable in the hybrid solar energy converter of FIG. 1;

FIG. 4 is a transverse cross-section along the line A-A of the solarenergy collector assembly of FIG. 3;

FIG. 5 is a longitudinal cross-sectional diagram along the line B-B ofthe solar energy collector assembly of FIG. 3;

FIG. 6 is a cut away view of the solar collector assembly of FIG. 3;

FIG. 7 is a diagram of a central sheet useable in the solar energycollector assembly of FIG. 3;

FIG. 8 is an explanatory diagram illustrating the operation of the solarenergy collector assembly of FIG. 3;

FIG. 9 is a transverse cross section along the line C-C of the solarenergy collector assembly of FIG. 3;

FIG. 10A is an explanatory diagram of the solar energy collectorassembly of FIG. 3;

FIG. 10B is an explanatory diagram of the solar energy collectorassembly of FIG. 3;

FIG. 11A is a detailed plan view of a part of the solar energy collectorassembly of FIG. 3;

FIG. 11B is a cross section along the line D-D of a the part of thesolar energy collector assembly of FIG. 3;

FIG. 12 is a diagram showing a part of the solar energy collectorassembly of FIG. 3 with the photovoltaic elements removed;

FIG. 13 is a diagram of a part of a solar energy collector assemblyaccording to a second embodiment according to the invention;

FIG. 14 is a diagram of a part of a solar energy collector assemblyaccording to a third embodiment according to the invention;

FIG. 15 is a diagram of a part of a solar energy collector assemblyaccording to a third embodiment according to the invention;

FIG. 16 is a diagram of a fourth embodiment of a hybrid solar energyconverter according to the invention;

FIG. 17 is a cut away diagram of a part of a solar energy collectorassembly useable in the hybrid solar energy converter of FIG. 16;

FIG. 18 is a diagram of a solar energy collector arranged for rotationabout a single axis;

FIG. 19 is a diagram of a solar energy collector array arranged forrotation about two axes;

FIG. 20 is a diagram of a solar energy collector arranged for rotationabout a single axis; and

FIG. 21 is a diagram of a solar energy collector array arranged forrotation about two axes.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Apparatus according to a first embodiment of the present invention isillustrated in FIG. 1. FIG. 1 shows a general exterior view of a firstembodiment of a hybrid solar energy converter 101 according to thepresent invention.

Overview

In the first embodiment, the hybrid solar energy converter 101 includesa solar energy collector assembly 102 housed within a sealed transparenttube 103. The solar energy collector assembly 102 includes an elongateheat transport element 104 and an array of photovoltaic elements 105mounted on an upper surface of the elongate heat transport element 104.The hybrid solar energy converter 101 also includes a support assembly106 at one end of the transparent tube 103. One end of the solar energycollector assembly 102 is connected to the support assembly 106. In oneexample the photovoltaic elements 105 may be formed of silicon. Inanother example the photovoltaic elements 105 may be formed of galliumarsenide. In other examples, photovoltaic elements of other suitablesemiconductor materials may be used. In other examples organicphotovoltaic elements may be used. In other examples hybrid photovoltaicelements may be used.

Photovoltaic elements may also be referred to as photovoltaic cells,solar cells or photoelectric cells. For the avoidance of doubt, in thepresent application the term photovoltaic element is used to refer toany element which converts incident electromagnetic radiation intoelectrical energy.

In the first embodiment, the heat transport element 104 includes a heatexchanger 107 arranged to transfer heat energy from the heat transportelement 104 to a first fluid. The heat exchanger 107 is located withinthe heat transport element 104 in the first embodiment, and accordinglyis not visible in FIG. 1.

In one possible example, in use the hybrid solar energy converter 101may be mounted on a roof. Accordingly, mounting brackets may beprovided.

An overview of the operation of the hybrid solar energy converter 101 ofthe first embodiment is that solar energy, in other words sunlight,incident on the hybrid solar energy converter 101 passes through thesealed transparent tube 103 and is incident on the photovoltaic elements105 of the solar energy collector assembly 102. The photovoltaicelements 105 convert a part of the energy of the incident solar energyinto electrical energy, and convert a part of the energy of the incidentsolar energy into heat energy. A further part of the incident solarenergy may be incident on any parts of the solar energy collectorassembly 102 which are not covered by the photovoltaic elements 105, andthis further part of the incident solar energy may also be convertedinto heat energy. In general, it is desirable to maximize the proportionof the surface of the solar energy collector assembly 102 exposed toincident solar energy which is covered by the photovoltaic elements 105,and to minimize the proportion which is not so covered. However, in somecircumstances it may be preferred to leave some parts of this exposedsurface uncovered, for example to simplify manufacture and/or assemblyof the solar energy collector assembly 102 and attachment of thephotovoltaic elements 105 to the solar energy collector assembly 102.Usually, in the first embodiment the surface of the solar energycollector assembly exposed to incident solar energy will be the uppersurface.

The electrical energy produced by the photovoltaic elements 105 iscarried along the heat transport element 104 by electrical conductorsand away from the solar energy converter 101 for use. The heat energyabsorbed by the photovoltaic elements 105 is transferred into the heattransport element 104, cooling the photovoltaic elements 105, and isthen transferred in the heat exchanger 107 to the first fluid. The firstfluid is supplied to the heat transport element 104 and the heatexchanger 107 through the support assembly 106, and the heated firstfluid travels out of the heat transport element 104 and through thesupport assembly 106, so that the heat energy in the heated first fluidcan be removed from the solar energy converter 101 and is available foruse.

In one typical arrangement, the hybrid solar energy converter 101 may beused in a domestic situation, such as on a household roof, to generateelectricity for household use and/or for export, and to generate hotwater for a domestic hot water and/or heating system. In thisarrangement the heat energy transferred to the first fluid in the heatexchanger 107 is used by a domestic or industrial hot water system, andthe electrical energy produced by the photovoltaic elements 105 issupplied to an electrical supply system. In some arrangements the firstfluid may be water.

Accordingly, the heat transport element 104 cools the photovoltaicelements 105. The efficiency of semiconductor photovoltaic elementsgenerally drops as the temperature of the semiconductor material rises.The temperature above which efficiency drops with increasing temperatureand the rate at which efficiency drops with increasing temperature willvary for different semiconductor materials and different designs ofphotovoltaic element. For silicon photovoltaic elements the efficiencyof electrical energy generation generally drops by about 0.35% to 0.5%for each degree centigrade of temperature increase above 25° C.

Transparent Tube

In the first embodiment illustrated in FIG. 1 the sealed transparenttube 103 is formed by a cylindrical glass tube having one open end 103 aand one closed domed end 103 b. The sealed transparent tube 103 isillustrated in more detail in FIG. 2. The open end 103 a of thecylindrical glass tube is sealed by a metal end cap 120 which is bondedto the glass tube with adhesive to form an air tight seal. The interiorof the tube 103 is at least partially evacuated. That is, the interiorof the tube is at a pressure below normal atmospheric pressure. Thepressure of the vacuum within the tube 103 may be 10⁻³ mbar.

The open end 103 a of the cylindrical glass tube sealed by the cap 120is attached to the support assembly 106 and the closed domed end 103 bis remote from the support assembly 106.

Insulated electrical conductors (not shown) pass through the metal cap120 to carry the electrical energy generated by the photovoltaicelements 105 away from the solar energy collector assembly 102.

As discussed above, the solar energy collector assembly 102 housedwithin the transparent tube 103 includes photovoltaic elements 105.Typically, photovoltaic devices are made from semiconductor materialswhich may suffer from oxidation and other environmental effectsadversely affecting their performance and lifetime when exposed to theatmosphere. The use of an evacuated tube 103 may protect thesemi-conductor materials of the photovoltaic elements 105 from suchenvironmental damage. This may allow the cost of encapsulating thephotovoltaic elements to be avoided.

The use of an evacuated tube may also increase the efficiency with whichheat can be collected from incident solar energy by the solar energycollector assembly 102. Having the solar energy collector assembly 102surrounded by an evacuated tube 103 may reduce or effectively preventconvective heat loss from the solar energy collector assembly 102 intothe material of the transparent tube 103 and the air around the hybridsolar energy converter 101.

Other vacuum pressures may be used. In some examples the vacuum pressuremay be in the range 10⁻² mbar to 10⁻⁶ mbar. In general, it is expectedthat lower vacuum pressure, or in other words a harder vacuum, willprovide greater insulating benefits. Further, it is expected that lowervacuum pressure, or in other words a harder vacuum, will provide greaterprotection from environmental damage in examples where the photovoltaicelements are not encapsulated. In practice the benefits of using a lowervacuum pressure may need to be balanced against the increased cost ofachieving a lower vacuum pressure. In some examples a vacuum pressure of10⁻² mbar, or lower, may be used.

In an alternative example the sealed transparent tube 103 may be filledwith an inert gas instead of being evacuated. In particular, the inertgas may be nitrogen.

In another alternative example the sealed transparent tube 103 may befilled with an inert gas at a reduced pressure. In some examples thismay be achieved by filling the tube 103 with the inert gas and thenevacuating the tube 103. In particular, the inert gas may be nitrogen.

In the illustrated first embodiment the tube 103 is cylindrical having acircular cross section. In alternative examples the tube 103 may haveother shapes. In some examples the cross sectional size and/or shape ofthe tube 103 may vary at different positions along its length. In analternative example the tube 103 may have an elliptical cross section.In particular, the tube 103 may have an elliptical cross section withthe long axis of the ellipse aligned with the plane of the solar energycollector assembly 102. The use of a tube 103 having an ellipticalcross-section with the long axis of the ellipse aligned with the planeof the solar energy collector assembly may reduce the amount of glassrequired by the tube 103 and may reduce reflection losses due to thereflection of incident solar energy from the tube 103.

In the illustrated first embodiment the tube 103 is formed of glass. Theuse of glass may allow the vacuum within the tube 103 to be maintainedlonger because the rate of migration of gas molecules from theatmosphere through glass is, in practice, effectively zero. Inalternative examples suitable transparent plastics materials orlaminated structures may be used to form the tube 103.

In the illustrated first embodiment the tube 103 is transparent. Inalternative examples the tube may be only partially transparent.

In the illustrated first embodiment the metal end cap 120 may be bondedto the glass tube 103 by adhesive. In other embodiments alternativeglass to metal bonding techniques may be used, for example welding,brazing or soldering.

In the illustrated first embodiment the tube 103 has a metal end cap 120at one end 103 a. In alternative examples the end cap 120 may be made ofother materials. In some examples the end cap 120 may be made of glass.This may reduce conductive heat losses from the collector assembly 102.

In the illustrated first embodiment the tube 103 has an end cap 120 atone end 103 a and one domed end 103 b. In alternative examples the tube103 may have an open end sealed by an end cap at both ends.

Collector Assembly

In the first embodiment, the solar energy collector assembly 102includes a heat transport element 104 and an array of photovoltaicelements 105 mounted on a surface of the heat transport element 104. Inorder to allow radiant solar energy to be incident on the photovoltaicelements 105 the array of photovoltaic elements 105 are mounted on thesurface of the heat transport element 104 which is exposed to theincident radiant solar energy in operation of the hybrid solar energyconverter 101. This will usually be the upper surface of the heattransport element 104.

In some arrangements the surface of the heat transport element 104exposed to the incident radiant solar energy may not be the uppersurface. In particular, this would be the case if the solar energycollector assembly 102 was located in a vertical, or substantiallyvertical, plane, or if the incident solar radiant energy was incidenthorizontally or from below, for example after redirection by an opticalsystem, such as a mirror. Accordingly, references to upper and lowersurfaces, and similar directional terminology in this description,should be understood as referring to the situation illustrated in thefigures where the solar energy collector assembly is in a plane at anangle to the horizontal and radiant solar energy is incident from above.

In the illustrated example of the first embodiment, the solar energycollector assembly 102 is supported by a cylindrical tube 119 of theheat transport element 104. The cylindrical tube 119 passes through theend cap 120 and extends between the heat transport element 104 and thesupport assembly 106. Although two sections of cylindrical tube 119 canbe seen extending between the heat transport element 104 and the supportsection 106 in FIG. 1, these are both sections of the same cylindricaltube 119, as will be explained in more detail below.

Where the cylindrical tube 119 passes through the end cap 120 thecylindrical tube 119 is soldered to the end cap 120 to retain thecylindrical tube 119 in place and support the solar energy collectorassembly 102. In alternative examples the cylindrical tube 119 may besecured to the end cap 120 in other ways. In one example the cylindricaltube 119 may be welded to the end cap 120.

The supporting of the solar energy collector assembly 102 by a physicalconnection through the cylindrical tube 119 may increase the efficiencywith which heat can be collected from incident solar energy by the solarenergy collector assembly 102. Having the solar energy collectorassembly 102 supported by a single physical connection through thecylindrical tube 119 may reduce conductive heat loss from the solarenergy collector assembly 102 into the supporting structure outside thetransparent tube.

In the first embodiment the heat transport element 104 has asubstantially flat upper surface 104 a. Each of the photovoltaicelements 105 is square, and the width of the heat transport element 104is the same as the width of each square photovoltaic element 105. Fivesquare photovoltaic elements 105 are mounted side by side to one anotheralong the length of the heat transport element 104. Substantially theentire upper face of the heat transport element 104 is covered by thephotovoltaic elements 105. Covering a large proportion of the uppersurface 104 a of the heat transport element 104 with photovoltaicelements 105 may increase the efficiency of the hybrid solar energyconverter 101.

In one example the square photovoltaic elements 105 may each be a 125 mmby 125 mm square and 0.2 mm thick. In another example the squarephotovoltaic elements may each be a 156 mm by 156 mm square. In otherexamples, photovoltaic elements having other sizes or shapes may beused.

The photovoltaic elements 105 are bonded to the substantially flat uppersurface 104 a of the heat transport element 104 using a layer 149 ofheat conducting adhesive in a similar manner to the first embodiment.This thermally conductive adhesive bonding layer 149 is shown in FIG. 3.The adhesive bonding layer 149 is electrically insulating. The adhesivebonding layer 149 between the photovoltaic elements 105 and the heattransport element 104 is arranged to be thin. This may improve thedegree of thermal conduction between the photovoltaic elements 105 andthe heat transport element 104. This may increase the rate of heattransfer laterally across the photovoltaic elements 105. An adhesivematerial loaded with solid spheres of a predetermined size may be usedto form the adhesive bonding layer 149. This may allow a thin adhesivelayer 149 to be consistently and reliably formed. The adhesive bondinglayer 149 is formed of a flexible or “forgiving” adhesive material. Thismay relieve stresses in the assembled solar energy collector assembly102 and reduce any stress applied to the photovoltaic elements 105.

The photovoltaic elements 105 are semiconductor photovoltaic elementsformed of silicon. In one example the photovoltaic elements are formedof single-crystal silicon. In one example the photovoltaic elements areformed of amorphous silicon. In one example the photovoltaic elementsare formed of polycrystalline silicon, or polysilicon. In other examplesalternative types of semiconductor photovoltaic elements may be used.

As discussed above, in operation of the hybrid solar energy converter101 the photovoltaic elements 105 are cooled by the heat transportelement 104. This cooling may allow the temperature of the photovoltaicelements 5 to be maintained at a desired value.

This cooling may provide the advantage that the appearance of hot spotsor regions in the photovoltaic elements 105 can be reduced oreliminated, and the temperature of the photovoltaic elements 105maintained at a uniform desired value. Such hot spots or regions may forexample be produced by heating by incident solar radiation, byinhomogeneities or faults in the photovoltaic elements 105, or by acombination of, or interaction between, these causes.

Such hot spots or regions can reduce the efficiency of the photovoltaicelements 105. It is believed that hot spots in the photovoltaic elements105 may reduce the efficiency of the photovoltaic elements 105 in theshort term, and may also degrade the performance of the photovoltaicelements 105 in the longer term. As discussed above, the efficiency ofphotovoltaic elements reduces as the temperature increases. In the shortterm a hot spot in a photovoltaic element may reduce the output of thephotovoltaic element because the material forming the hot spot is at ahigher temperature than the rest of the photovoltaic element, and so hasa reduced efficiency compared to the rest of the photovoltaic element.Further, in the longer term the degrading of the performance of thephotovoltaic element may also take place more rapidly at a hot spotbecause the material forming the hot spot is at a higher temperaturethan the rest of the photovoltaic element.

Accordingly, maintaining the photovoltaic elements 105 at a more uniformtemperature value and reducing, or eliminating, hot spots or regions mayimprove the efficiency of the photovoltaic elements 105 at a specifictemperature, and may reduce the amount of degradation of thephotovoltaic elements 105 caused by higher temperatures.

This may allow the photovoltaic elements 105 to operate at a higheroverall temperature than would otherwise be the case. This may beunderstood by considering that where hot spots exist in the photovoltaicelements 105 it may be the temperature induced reduction in efficiencyand temperature induced degradation in these hot spots that limits themaximum operating temperature of the photovoltaic element 105 as awhole. As a result, reducing, or eliminating, these hotspots may allowthe maximum operating temperature of the photovoltaic element 105 as awhole to be raised.

The illustrated example of the first embodiment has a solar energycollector assembly 102 supported by a physical connection through thecylindrical tube 119. In other examples alternative supportingarrangements may be used. In some examples the solar energy collectorassembly 102 may be supported by physical connections at each end of thesolar energy collector assembly 102. In some examples, the physicalconnection at one end of the solar energy collector assembly may be thethrough the cylindrical tube. In general, it is advantageous to minimizethe number of physical supports in order to minimize the escape of heatfrom the solar energy collector assembly by conduction through thephysical supports.

In other examples the number of photovoltaic elements 105 mounted on theheat transport element 104 may be different. In other examples therelative sizes of the photovoltaic elements 105 and the heat transportelement 104 may be different.

In some examples the adhesive layer 149 may comprise an epoxy resinwhich remains non-brittle after curing.

In other examples the adhesive layer 149 may be formed by a double sidedadhesive tape.

Heat Transport Element

The heat transport element 104 according to the first embodiment isshown in more detail in a cut away view in FIG. 3, and in transverse andlongitudinal cross-sectional views in FIGS. 4, and 5 respectively. Thetransverse cross section of FIG. 4 is taken along the line A-A in FIG.3. The longitudinal cross section of FIG. 5 is taken along the line B-Bin FIG. 3.

In the second embodiment, the elongate heat transport element 104 isgenerally rectangular. The heat transport element 104 has a flat uppersurface 104 a and a lower surface 104 b which is flat across most of itsarea, and has an outwardly projecting section 110 along one edge 104 cof the heat transport element 104. The outwardly projecting section 110contains and defines an elongate heat transfer chamber or vapor manifold111 extending along substantially the entire length of the elongate heattransport element 104. In operation the heat transport element 104 isarranged to be transversely sloping, so that the side edge 104 c of theheat transport element 104 bearing the outwardly projecting section 110is higher than the opposite side edge 104 d of the heat transportelement 104, for reasons which will be explained in detail below. Theinclination angle of the heat transport element 104 to the horizontalmay be small. An inclination of about 5° is sufficient. Larger angles ofinclination may be used if desired. An angle of inclination up to andincluding 90° may be used, i.e. the heat transport element 104 may bearranged transversely vertically.

FIG. 6 shows a cut away plan view from below of the heat transportelement 104 of FIG. 3 with the outwardly projecting section 110 removed,so that the heat exchanger 107 is visible.

As can be best seen in FIG. 6, the heat exchanger 107 is comprises acylindrical tube 119 which extends within the elongate heat transferchamber or vapor manifold 111 along substantially the entire length ofthe elongate heat transfer chamber or vapor manifold 111. Thecylindrical tube 119 comprises first and second parallel straightsections 119 a and 119 b, each parallel to the sides of the vapormanifold 111, such as the upper surface 104 a and the projecting section110, and extending along substantially the entire length of the vapormanifold 111. The two straight sections 119 a and 119 b are connectedtogether by a curved section 119 c.

In use, the first fluid passes along the cylindrical tube 119 asindicated by the arrows in FIG. 6, so that the first fluid passesthrough the first straight section 119 a, the curved section 119 c andthe second straight section 119 b, so that the first fluid passesthrough the cylindrical tube 119, and travels along substantially theentire length of the vapor manifold twice.

In the illustrated example the first and second sections 119 a and 119 bof the cylindrical tube 119 are arranged one above the other and thefirst fluid enters through the first section 119 aa and leaves throughthe second section 119 b. In other examples the direction of flow of thefirst fluid may be reversed. In other examples the first and secondsections 119 a and 119 b may be differently arranged.

The heat transport element 104 has an upper surface 104 a formed by anupper sheet 114 and a lower surface 104 b formed by a lower sheet 115. Acentral sheet 116 is located between the upper sheet 114 and the lowersheet 115, so that fluid flow passages 117 and 118 running transverselyacross the heat transport element 104 are defined between the centralsheet 116 and each of the upper sheet 114 and the lower sheet 115. Thefluid flow passages 117 and 118 are sloped along their lengths. In theillustrated example the heat transport element 104 is transverselysloping, and as a result the fluid flow passages 117 and 118 runningtransversely across the heat transport element 104 will be sloped alongtheir lengths.

FIG. 5 shows the profile of the central sheet 116 in more detail. FIG. 5shows a longitudinal cross section along the line B-B in FIG. 3. Thecentral sheet 116 is formed with a corrugated profile having ridges andtroughs which run transversely across the heat transport element 104.The cross-sectional profile of the corrugated central sheet 116 can beunderstood as a zig-zag profile with the points of the zig-zag formingthe peaks and troughs being flattened. Accordingly, the upper and lowerfluid flow passages 117 and 118 are interleaved. The upper and lowerfluid flow passages 117 and 118 are arranged side by side in a planararray with upper fluid flow passages 117 and lower fluid flow passages118 arranged alternately.

To be more specific, in the illustrated example of the first embodimentthe central sheet 116 comprises a plurality of flat surfaces connectedby folds running transversely across the heat transport element 104. Asshown in FIG. 7, the central sheet 116 comprises a first series of firstcoplanar surfaces 116 a spaced apart equidistantly in a first plane Cand a second series of second coplanar surfaces 116 b spaced apartequidistantly in a second plane D, each of the first and second coplanarsurfaces 116 a and 116 b having the same width, and the separationbetween successive coplanar surfaces 116 a or 116 b of each of the firstand second series of first and second coplanar surfaces 116 a and 116 bbeing larger than the width of the coplanar surfaces 116 a and 116 b.The first and second planes C and D are parallel and spaced apart. Thefirst and second series of coplanar surfaces are arranged so that inplan view, i.e. when viewed perpendicularly to the first and secondplanes C and D, each of the first coplanar surfaces 116 a is locatedequidistantly between two of the second coplanar surfaces 116 b, andvice-versa. The first and second coplanar surfaces 116 a and 116 b areinterconnected by a first series of first parallel linking surfaces 116c and a second series of second parallel linking surfaces 116 d.

As is shown particularly in FIG. 5, the central sheet 116 is arrangedwith the first surfaces 116 a contacting an inner face of the uppersheet 114 and the second surfaces 116 b contacting an inner face of thelower sheet 115. The first surfaces 116 a of the central sheet arebonded to the upper sheet 114 and the second surfaces 116 b of thecentral sheet 116 are bonded to the lower sheet 115. Accordingly, theupper lower, and central sheets 114, 115, 116 define a plurality oftrapezoid cross-section upper fluid flow channels 117 and lower fluidflow channels 118 between them. The upper fluid flow channels 117 aredefined between the upper sheet 114 and the central sheet 116. The lowerfluid flow channels 118 are defined between the lower sheet 115 and thecentral sheet 116. The trapezoid upper fluid flow channels are arrangedso that the larger one of the two parallel faces of the trapezoidchannel is formed by the upper sheet 114.

The edges of the heat transport element 104 are formed by upwardly bentparts of the lower sheet 115, which are bonded to the upper sheet 114.The photovoltaic elements 105 are bonded to the upper sheet 114. At theedges of the heat transport element 104, the upper sheet 114 is bondeddirectly to the lower sheet 115, the central sheet 116 is not locatedbetween the upper and lower sheets 114 and 115 at their edges.

In some examples the central sheet 116 may extend at least partiallybetween the upper and lower sheets 114 and 115 at the end edges of theheat transport element 104 so that the upper and lower sheets 114 and115 are both bonded to the central sheet 116. This may assist inlocating and securing the central sheet 116 relative to the upper andlower sheets 114 and 115.

As discussed above, the heat transport element 104 has an outwardlyprojecting section 110 along the upper side edge 104 c of the heattransport element 104. The outwardly projecting section 110 issubstantially semi-cylindrical and is formed by an outwardly projectingpart of the lower sheet 115. The outwardly projecting section 110defines a vapor manifold 111. The fluid flow channels 117 and 118connect to the vapor manifold 111. It should be noted that the centralsheet 116 extends across most of the width of the vapor manifold 111.Accordingly, the upper fluid flow channels 117 defined between the uppersheet 114 and the central sheet 116 connect to the vapor manifold 111towards the top of the vapor manifold 111, while the lower fluid flowchannels 118 defined between the lower sheet 115 and the central sheet116 connect to the vapor manifold 111 towards the bottom of the vapormanifold 111. All of the upper and lower fluid flow channels 117 and 118are interconnected by the vapor manifold 111.

At the lower side edge 104 d of the heat transport element 104 oppositethe outwardly projecting section 110, there is a gap 123 between theedge of the central sheet 116 and the side edge 104 c of the heattransport element 104 formed by an upwardly bent part of the lower sheet115. This gap 123 allows water to flow between different ones of thefluid flow channels 117 and 118. The gap 123 extends along the side edge104 d of the heat transport element 104, and forms a fluid manifold 124interconnecting all of the upper and lower fluid flow channels 117 and118.

At each end of the heat transport element 104 the substantiallysemi-cylindrical outwardly projecting section 110 extending most of thelength of the heat transport element 104 is closed by an end wall 108.The upper and lower sheets 114 and 115 are sealed by the end walls 108so that the interior of the heat transport element 104 is sealed. Thecylindrical tube 119 passes through the end wall 108 adjacent the openend of the glass tube 103 and the end cap 120, through the end cap 120and into the support assembly 106. The cylindrical tube 119 within theheat transfer chamber or vapor manifold 111 forms the heat exchanger 107and acts to carry heat energy from the heat transport element 104 awayfrom the hybrid solar energy converter 101, as will be explained below.

The cylindrical tube 119 physically supports the solar energy collectorassembly 102 within the sealed transparent tube 103. There is no otherphysical support of the solar energy collector assembly 102. This mayreduce conductive heat losses from the solar energy collector assembly102, which may increase the amount of useful heat energy produced by thehybrid solar energy converter 101.

The fluid flow channels 117 and 118 are at least partially filled withdegassed distilled water 121 as a working fluid and the interior of theheat transport element 104 including the fluid flow channels 117 and118, and the vapor manifold 111 are at least partially evacuated. Thatis, the interior of the heat transport element 104 is at a pressurebelow normal atmospheric pressure. The interior of the heat transportelement 104 may be under a vacuum at a pressure of 10⁻³ mbar. The heattransport element 104 is arranged to be laterally inclined to thehorizontal with the side 104 a of the heat transport element 104 wherethe vapor manifold 111 is located being arranged to be higher than theopposite side 104 b of the heat transport element 104.

In the illustrated first embodiment the amount of water 121 in the fluidflow channels 117 and 118 is such that an upper surface 132 of the water121 in the lower fluid flow channels 118 is level with the ends of thelower fluid flow channels 118 where the lower fluid flow channels 118connect to the vapor manifold 111. In the illustrated second embodimentthe level of the surface 132 of the water 121 in the upper fluid flowchannels 117 and lower fluid flow channels 118 is substantially thesame. Accordingly, in the illustrated second embodiment the lower fluidflow channels are filled with liquid water, while the upper fluid flowchannels 117 are only partially filled with liquid water.

In other examples the level of the water 121 may be different. In someexamples the upper surface 132 of the water 121 in the lower fluid flowchannels 118 may be below the vapor manifold 111. In some examples theupper surface 132 of the water 121 in the lower fluid flow channels 118may be above the bottom of the vapor manifold 111, with some water beingpresent in the bottom of the vapor manifold 111.

It is expected that in practice the heat transport element 104 willoperate most efficiently with the upper surface 122 of the water beingat, or close to, the point where the lower fluid flow channels 118contact the vapor manifold 111. If the level of the water in the heattransport element 104 is too high, so that the upper surface 122 of thewater is too high within the vapor manifold 111, the efficiency ofoperation of the heat transport element 104 may be reduced, as will bediscussed in more detail below.

The upper surface 132 of the water 121 in the upper fluid flow channels117 may be higher than in the lower fluid flow channels 118 as a resultof capillary action. The extent of this capillary effect in any specificexample will depend upon the dimensions of the upper fluid flow channels117. In the illustrated first embodiment some of the inner surface ofthe upper sheet 114, that is, the surface forming a part of the upperfluid flow channels 117, is above the surface of the water 121. In someexamples the upper fluid flow channels 117 may have a small enoughcross-sectional area that the upper surface 123 of the water 121 in theupper fluid flow channels 117 is at the ends of the upper fluid flowchannels 117 due to capillary action.

It should be noted that it is not necessary that the inner surface ofthe upper sheet 114, that is, the surface forming a part of the upperfluid flow channels 117, is below the upper surface 132 of the water 121at a position corresponding to the location of the uppermost parts ofthe photovoltaic elements 105. However, in some examples this may be thecase.

In operation of the first embodiment, when the solar energy collectorassembly 102 is exposed to incident solar radiative energy, thephotovoltaic elements 105 absorb some of this energy, converting a partof the absorbed energy into electrical energy. The remainder of theabsorbed energy is converted into heat energy, raising the temperatureof the photovoltaic elements 105. The absorbed heat energy flows fromthe photovoltaic elements 105 into the heat transport element 104, beingtransmitted through the upper sheet 114 and into the water 121 insidethe upper fluid flow channels 117, which water is in contact with theinner surface of the upper metal sheet 114 across the larger parallelfaces of the trapezoid upper fluid flow channels 117.

The liquid water 121 inside the upper fluid flow channels 117 absorbsthe heat energy from the photovoltaic elements 105 passing through theupper sheet 114 and vaporizes, producing bubbles 122 of steam or watervapor, as shown in FIG. 8. The liquid water may vaporize and producebubbles as a result of either or both of convection boiling andnucleation. At the vacuum pressure of 10⁻³ mbar inside the upper fluidflow channels 117 water boils from around 0° C., so that the water 121vaporizes readily at the normal operating temperatures of the hybridsolar energy converter 101.

The bubbles 122 of water vapor are less dense than the liquid water 121.Further, as explained above the upper fluid flow channels 117 aresloping along their lengths. Accordingly, as a result of this densitydifference the water vapor bubbles 122 travel upwards along the upperfluid flow channels 117 towards the upper side edge 104 c of the heattransport element 104 and the surface of the water 121. When a bubble122 of water vapor reaches the surface of the water 121 the vapor isreleased into the vacuum above the water 121 in the vapor manifold 111.Further, as a bubble 122 travels upwards along a fluid flow channel 117the bubble 122 will act as a piston to drive the liquid water, and anyother bubbles 122 above it, upwardly along the upper fluid flow channel117. This pistonic driving may tend to accelerate the speed with whichthe vapor bubbles 122 move upward along the upper fluid flow channels117. This pistonic driving may act to pump liquid water upwards alongthe upper fluid flow channels 117 to the ends of the upper fluid flowchannels 117, where the liquid water will be ejected from the upperfluid flow channels 117 into the vapor manifold 111. In the illustratedfirst embodiment, where some of the inner surface of the upper sheet 114is above the surface of the water 121, this pumping of liquid waterupwards along the upper flow channels 117 ensures that the part of theinner surface of the upper sheet 114 above the surface of the water 121is in contact with a flow of water so that it can be cooled.

The amount of the pistonic driving produced by the bubbles 122 willdepend upon the relative sizes of the bubbles 122 compared to thecross-sectional areas of the upper fluid flow channels 117. The amountof pistonic driving produced by the bubbles 122 may be increased wherethe size of the bubbles is relatively large compared to thecross-sectional areas of the upper fluid flow channels 117. The pistonicdriving produced by the bubbles 122 may be particularly effective inexamples where the size of the bubbles 122 of water vapor is equal to,or only a little smaller than, the cross sectional areas of the upperfluid flow channels 117.

In practice the sizes of individual water vapor bubbles will vary.However, the likely average sizes of the bubbles and the likelyvariability in their sizes can be determined in any specific case, basedon the operating parameters to be used in the hybrid solar energyconverter.

The bursting of the bubbles of water vapor at the water surface and anypistonic pumping of liquid water out of the ends of the upper fluid flowchannels 117 may generate droplets of liquid water, and may project atleast some of these water droplets into the vacuum within the vapormanifold 111 above the water surface. As a result, the heat transfermechanism may be a multi-phase system comprising liquid water, watervapor and droplets of liquid water, and not just a two-phase systemcomprising liquid water and water vapor only. The presence of suchdroplets of water in the vacuum, and any pumping of liquid water out ofthe ends of the upper fluid flow channels 117, may enhance the rate ofvaporization by increasing the surface area of the water exposed to thevacuum.

The water vapor in the vacuum within the vapor manifold 111 travels at avery high speed through the vacuum within the vapor manifold 111 intocontact with the exterior surface of the tube 119 forming the heatexchanger 107. The travel speed of the hot water vapor in the vacuum isvery fast, approximating to the thermal speed of the water vapormolecules. The water vapor condenses on the external surface of the tube119, which acts as a heat exchange surface. The condensed water fallsoff the tube 119 to the bottom of the vapor manifold 111, and isreturned back into the water 121 within the lower fluid flow channels118. This generating of hot water vapor within the upper fluid flowchannels 117 and the vapor manifold 111, and the condensing of the hotwater vapor on the tube 119, followed by return of the condensed waterinto the fluid flow channels 117 and 118, transfers heat energy from theheat transfer element 104 to the first fluid within the tube 119.

Thus, the working fluid is caused to circulate on a working fluidpathway inside the heat transfer device such that the heat of theworking fluid is transferred to the heat exchanger in the heat transferchamber. In the illustrated example the working fluid pathway is formedby the fluid flow channels 117 and 118 and the heat of the working fluidis absorbed from the cooled surface 104 a of the heat transport element104.

The first fluid flows from the support element 106, through the heattransfer element 104, where it absorbs heat, and the heated fluid thenreturns to the support element 106. In the illustrated first embodimentthe first fluid is water and a pumped water supply passes through thesupport element 106 and the cylindrical tube 119.

Any liquid water ejected from the upper fluid flow channels 117 into thevapor manifold 111 which does not vaporize will also fall to the bottomof the vapor manifold 111, and is returned back into the water 121within the lower fluid flow channels 118.

The description indicates that water droplets and condensed water fallsinto the bottom of the vapor manifold 111. In some examples, dependingupon the orientation of the collector assembly 102, some or all of thiswater may fall into the lower fluid flow channels 118 withoutnecessarily contacting the surface of the vapor manifold 111.

The location of the heat exchanger 107 within the heat transfer chamberor vapor manifold 111 may improve the efficiency of the heat transferelement 104 by providing a short path for water vapor to travel betweenthe upper surface of the working fluid and the heat exchanger 107.

As is explained above, all of the upper and lower fluid flow channels117 and 118 are interconnected by the fluid manifold 124 formed by thegap 123. Accordingly, it is not important which of the lower fluid flowchannels 118 is entered by any liquid water returning from the vapormanifold 111.

As is clear from the description above, the heat transfer chamber orvapor manifold 111 generally includes liquid water in addition to watervapor when the hybrid solar energy converter 101 is operating. However,as is also discussed above, if the level of the water in the heattransport element 104 is too high, so that the upper surface 122 of thewater is too high within the vapor manifold 111, the efficiency ofoperation of the heat transport element 104 may be reduced. Thisreduction in efficiency of operation may occur because there isinsufficient space within the vapor manifold 111 above the surface ofthe water for the movement and evaporation of the droplets of liquidwater. This reduction in efficiency of operation may occur because thedroplets of liquid water and waves and splashing upwardly of the liquidwater surface may reduce the open, or water free, cross sectional areaof the vapor manifold 111 at some locations to a relatively smallamount, or even to zero, momentarily closing the vapor manifold 111.This reduction in the open, or water free, cross sectional area of thevapor manifold 111 may interfere with the movement of the water vapor inthe vacuum within the vapor manifold 111.

The bubbles 122 of water vapor will tend to move upwardly through theliquid water in the upper fluid flow channel 117 because of the lowerdensity of the water vapor compared to the liquid water 121, which willresult in an upward buoyancy force on each bubble 122. Further, themovement of the bubbles 122 of water vapor will tend to drive the liquidwater 121 in the upper fluid flow channel 117 upwardly, particularly inexamples where pistonic driving takes place. As a result, the bubbles122 of water vapor cause the water 121 in the upper and lower fluid flowchannels 117 and 118 to circulate, with relatively hot liquid water andbubbles 122 of water vapor flowing upwards along the upper fluid flowchannels 117, and relatively cool liquid water flowing downwards alongthe lower fluid flow channels 118. The upper and lower fluid flowchannels 117 and 118 are interconnected by the vapor manifold 111 andthe fluid manifold 124, as explained above. Accordingly, the relativelyhot liquid water flowing upwards along the upper fluid flow channels iscontinuously replaced by relatively cool liquid water from the lowerfluid flow channels 118. This circulation is driven primarily by thedifference in density between the water vapor and the liquid water.However, this circulation may also be driven by convection as a resultof the difference in density between the relatively hot liquid water inthe upper fluid flow channels 117 and the relatively cool liquid waterin the lower fluid flow channels 118, in a similar manner to athermosiphon. Accordingly, the upper fluid flow channels 117 may beregarded as riser channels, while the lower fluid flow channels 118 maybe regarded as sinker channels or return channels.

As the bubbles 122 of water vapor travel upwardly along the upper fluidflow channels 117 the pressure head acting on the bubbles 122 decreases,so that the bubbles 122 tend to expand. As a result, the tendency of thevapor bubbles 122 to collapse and implode is reduced by the effects ofthe expansion and decreasing pressure as the bubbles 122 move upwardly.When considering this point, it should be remembered that when the heattransport element 104 is operating the bubbles 122 will be formingwithin an established density driven circulation fluid flow and willmove upwardly carried by this flow in addition to the bubbles movementdue to their own buoyancy relative to the liquid water. Further, it isbelieved that expansion of the bubbles 122 as they move upwardly willfurther increase the speed of the density driven circulation flow byincreasing the buoyancy of the expanding bubbles 122. In some examplesexpansion of the bubbles as they move upwardly may also increase thedegree of pistonic driving.

This density driven circulation may form a highly effective heattransport mechanism because water has a relatively high enthalpy ofvaporization, so that the movement of the bubbles 122 of water vapor maycarry a large amount of heat energy, in addition to the heat energycarried by the movement of relatively hot water out of the upper fluidflow channels 117, and its replacement by cooler water. In arrangementswhere pistonic driving of the flow of the liquid water by the watervapor bubbles takes place the effectiveness of the heat transportmechanism may be further increased by the increase in the flow rate ofthe liquid water caused by the pistonic driving. This pistonic drivingis a component of the overall density driving producing the densitydriven circulation. The pistonic driving is caused by the densitydifference between the liquid water and the bubbles of water vapor.

In general, the speed of the density driven circulation increases andthe effectiveness of the heat transport mechanism increases as thetemperature of the upper sheet 114 of the heat transport element 104increases.

The density driven circulation of the water 121 within the fluid flowchannels 117 and 118 is a vapor driven circulating or rolling flow.

The density driven circulation of the water 121 within the fluid flowchannels 117 and 118 becomes particularly vigorous, and becomesparticularly effective as a heat transport mechanism, when thetemperature of the upper sheet 114 of the heat transport element 104becomes sufficiently high that the water 121 within the fluid flowchannels 117 and 118 enters a rolling boil state. The effectiveness ofthe heat transport mechanism significantly increases when rollingboiling of the water 121 commences. In general, when other parameters ofthe system remain constant, entry into the rolling boil state will takeplace when the temperature of the upper sheet 114 of the heat transportelement 104 reaches a specific temperature.

In the illustrated example using water, the water 121 within fluid flowchannels 117 and 118 may enter a rolling boil state at a temperature ofabout 40° C.

The arrangement of fluid flow channels 117 extending laterally acrossthe heat transport element 104 may allow the vertical height of theliquid water in the heat transport element 104 to be reduced compared toarrangements in which the density driven flow extends along the lengthof a heat transport element, and so reduce the pressure head acting onthe liquid water at the bottom of the heat transport element 104. Ingeneral, increased pressure reduces the tendency of liquids to vaporizeand so increases the boiling point of liquids. Accordingly, reducing thepressure head acting on the liquid water at the bottom of the heattransport element 104 may increase the tendency of the liquid water 121towards the lower ends of the upper fluid flow channels 117 to vaporizeand produce bubbles 122, and so may improve the efficiency andeffectiveness of the heat transport element 104.

In particular, the reduction of the pressure head acting on the liquidwater at the bottom of the upper fluid flow channels 117 may reduce anytemperature differential along the lengths of the upper fluid flowchannels between their the top and bottom ends by reducing anydifference in the tendency of the liquid water to vaporize due todifferences in pressure. This may reduce temperature differentialsbetween the different points on the heat transport element 104 and mayassist in reducing or avoiding the formation of hot spots in thephotovoltaic elements 105.

In general the forming of hot spots in the photovoltaic elements 105 isundesirable because these may lead to a reduction in the efficiency ofelectrical energy generation in the photovoltaic elements 105, whichreduction in efficiency may be permanent.

The arrangement of the upper fluid flow channels 117 extending laterallyacross the heat transport element 104 and interconnected by a heattransfer chamber or vapor manifold 111 containing a heat exchanger 107extending longitudinally along the heat transport element 104 may allowa very rapid flow of heat energy along the heat transport element 104away from any upper fluid flow channel 117 having a higher temperature.This may reduce temperature differentials between the different pointson the heat transport element 104 and may reduce, or avoid, theformation of hot spots in the photovoltaic elements 105.

The provision of the two separate heat transport mechanisms of themovement of water vapor along the heat transfer chamber or vapormanifold 111 and the density driven flow of liquid water and water vaporalong each of the upper fluid flow channels 117, respectively actinglongitudinally and transverse the length of the heat transport element104, may tend to equalize the temperature across the entire uppersurface of the heat transport element, and thus tend to equalize thetemperature across the photovoltaic elements 105 and reduce, or avoid,the formation of hot spots.

The movement of water vapor along the heat transfer chamber or vapormanifold 111 provides a very rapid heat transport mechanism that tends,by the vaporization and condensation of water, to move heat energy fromrelatively hot locations to relatively cold locations. As a result, themovement of water vapor along the heat transfer chamber or vapormanifold 111 may tend to equalize the temperature of the liquid watersurface at different positions along the heat transfer element 104, inaddition to transporting heat energy from the heat transport element104, and specifically from the upper surface 104 a of the heat transportelement 104, to the heat exchanger 107 formed by the tube 119. Thistemperature equalization may have the effect of removing more heatenergy from hotter parts of the upper surface 104 a of the heattransport element 104, and so tending to equalize the temperature acrossthe upper surface 104 a. It is clear that such isothermal cooling willtend to reduce, or avoid, the formation of hot spots, for example in anyphotovoltaic element attached to the upper surface 104 a.

The lower sheet 115 of the heat transport element 104 has a plurality ofhollow ridges 125 extending between the flat part of the lower surface104 b and the semi-cylindrical surface of the outwardly projectingsection 110. Each hollow ridge 125 has a ‘V’ profile, and the hollowridges 125 are located spaced apart at regular intervals along thelength of the heat transport element 104. FIG. 9 shows a transversecross section of the heat transport element 104 taken along the line C-Cin FIG. 3. The line C-C of FIG. 9 is parallel to the line A-A of FIG. 4,but passes through one of the hollow ridges 125. The hollow ridges 125act as supports for the outwardly projecting section 110, acting asbuttresses and helping to keep the curved part of the lower sheet 115forming the outwardly projecting section 110 fixed relative to the flatpart of the lower metal sheet 115 and the other parts of the heattransport element 104.

The hollow ridges 125 also act as drains to return liquid water from thevapor manifold 111 into the lower fluid flow channels 118, as will beexplained in more detail below.

As explained above, the vapor manifold 111 is semi-cylindrical, beingdefined by the semi-cylindrical outwardly projecting section 110 formedby a curved part of the lower sheet 115. Further, as explained above,the heat transport element 104 is transversely sloping so that the sideedge 104 c of the heat transport element 104 bearing the outwardlyprojecting section 110 is higher than the other side edge 104 d of theheat transport element 104. As a result, depending upon the transverseinclination angle of the heat transport element 104 there may, or maynot, be parts of the vapor manifold 111 which are located below the endsof the lower fluid flow channels 118 where the lower fluid flow channels118 connect to the vapor manifold 111.

FIGS. 10A and 10B are explanatory diagrams, each showing a transversecross sectional view of the heat transport element 104 corresponding tothe view shown in FIG. 4. FIG. 10A shows the heat transport element 104inclined at a relatively large angle to the horizontal, while FIG. 10Bshows the heat transport element 104 inclined at a relatively smallangle to the horizontal.

When the heat transport element is inclined at a relatively small angleto the horizontal, as shown in FIG. 10A, the lower fluid flow channels118 connect to the vapor manifold 111 at the lowest point of thesemi-cylindrical outwardly projecting section 110 of the lower sheet 115defining the vapor manifold 111. In this position all liquid waterwithin the vapor manifold 111 will drain directly into the lower fluidflow channels 118. In contrast, when the heat transport element 104 isinclined at a relatively large angle to the horizontal, as shown in FIG.10B, the part of the semi-cylindrical outwardly projecting section 110of the lower sheet 115 defining the vapor manifold 111 is located belowthe point at which the lower fluid flow channels 118 connect to thevapor manifold. In this position, in the absence of the hollow ridges125, some liquid water within the vapor manifold 111, specificallyliquid water below the horizontal line 126, could be retained within thevapor manifold 111 and not drain into the lower fluid flow channels 118.

The hollow ridges 125 form a drain path for liquid water in the vapormanifold 111 to return to the lower fluid flow channels 118 and soprevent the retention of a reservoir of liquid water within the vapormanifold 111 which might otherwise occur.

As discussed above, the heat transport assembly 104 can operate withliquid water within the vapor manifold 111. However, in the absence ofthe hollow ridges 125 the existence and size of any reservoir of liquidwater retained in the vapor manifold 111 will vary depending on theangle of inclination to the horizontal of the heat transport element104, and the resulting changes in the liquid water level in the fluidflow channels 117 and 118 at different angles of inclination mayadversely affect the operation of the heat transport element 104 at someangles of inclination and so limit the range of angles of inclination atwhich the heat transport element 104 can be used.

Accordingly, the hollow ridges 125 may extend the range of angles ofinclination at which the heat transport element 104 can be used.

Depending upon the geometry of the different parts of the heat transportelement 104 in any specific design, even when the hollow ridges 125 areused there may still be a minimum angle of inclination at which the heattransport element 104 can operate without the retention of liquid waterin the vapor manifold 111 having adverse effects on operation of theheat transport element 104.

In the illustrated example of the first embodiment the hollow ridges 125act as supports for the outwardly projecting section 110 and also act asdrains to return liquid water from the vapor manifold 111 into the lowerfluid flow channels 118. In some examples these functions may be carriedout by separate dedicated structures.

The corrugated profile of the central sheet 116 and the bonding of thefirst and second surfaces 116 a and 116 b of the central sheet 116 tothe upper sheet 114 and the lower sheet 115 so that the linking surfaces116 c and 116 d of the central sheet 116 interconnect the upper andlower sheets 114 and 115 increases the strength and rigidity of the heattransport element 104. This may make the heat transport element 104 amore rigid structure. This may tend to reduce the amount of flexing ofthe heat transport element 104 in use. This may prevent damage to thephotovoltaic elements 105 by reducing the amount of mechanical stressapplied to the photovoltaic elements 105. This may allow the upper,lower, and/or central metal sheets 114, 115, 116, to be thinner, whichmay reduce weight and costs. This may allow the upper metal sheet 114 tobe thinner, which may improve the transfer of heat from the photovoltaicelements 105 into the liquid water within the upper fluid flow channels117.

The heat transport element 104 is a substantially rigid structure. Thismay minimize changes in the level of the upper surface 132 of the water121 due to flexing of the components of the heat transport element 104,such as the upper and lower sheets 114 and 115. Such changes in thelevel of the upper surface 132 of the water 121 may affect theefficiency of the cooling of the photovoltaic elements 105.

As is explained above, the interior of the heat transport element 104 isevacuated, and the heat transport element 104 is located within anevacuated tube 103. Usually the heat transport element 104 and theevacuated tube 103 are evacuated to the same pressure. In theillustrated example of the second embodiment described above thispressure may be 10⁻³ mbar.

When the water within the heat transport element 104 is heated theproportion of the water in a vapor phase will increase and theproportion in a liquid phase will decrease. As a result the pressurewithin the heat transport element 104 will increase, producing apressure differential between the interior and exterior of the heattransport element 104. This pressure differential may cause the upperand lower metal sheets 114 and 115 to ‘balloon’, or bend outwards. Theinterconnection of the upper and lower metal sheets 114 and 115 by thelinking surfaces 116 c and 116 d of the central metal sheet 116 mayresist such ballooning of the upper and lower metal sheets 114 and 115and reduce or prevent ballooning. Arranging for the linking surfaces 116c and 116 d of the central metal sheet 116 to be straight may increasethe resistance to ballooning. Reducing or preventing ballooning mayprevent damage to the photovoltaic elements 105 by reducing the amountof mechanical stress applied to the photovoltaic elements 105. This mayallow the upper metal sheet 114 to be thinner, which may reduce weightand costs and/or may improve the transfer of heat from the photovoltaicelements 105 into the liquid water within the upper fluid flow channels117.

The above description of the operation of the heat transfer element 104according to the second embodiment describes the transfer of heat energyfrom the photovoltaic elements 105 through the upper metal sheet 114 andinto the water within the upper fluid flow channels 117. In addition, inthe regions of the upper metal sheet 114 bonded to the first surfaces116 a, some heat energy will pass through the upper metal sheet 114 andthe central metal sheet 116 into the water within the lower fluid flowchannels 118. Although this transfer of heat energy will cool thephotovoltaic elements 105, the heating of the water in the lower fluidflow channels 118 is generally undesirable because it will tend tocounteract and slow the density driven circulation of water produced bythe heating of the water in the upper fluid flow channels 117 describedabove. Accordingly, it is preferred for the sizes of the first surfaces116 a of the central metal sheet 116 in contact with the upper metalsheet 114 to be as small as possible, subject to the contact areabetween the first surfaces 116 a and the upper metal sheet 114 beingsufficiently large to form a reliable bond of the required strength.

It is not necessary for the heat transport element 104 according to thefirst embodiment to be inclined to the horizontal along its longitudinalaxis. In other words, it is not necessary for the end of the heattransport element 104 adjacent the support assembly 106 to be higherthan the end of the heat transport element 104 remote from the supportassembly 106.

In the illustrated first embodiment the heat transport element 104 isarranged to be horizontal along its longitudinal axis. That is, the endof the heat transport element 104 adjacent the support assembly 106should be at the same height as the end of the heat transport element104 remote from the support assembly 106. However, in practice somedeviation from the horizontal may be tolerated without significantimpact on the operation of the heat transport element 104. Suchdeviation from the horizontal will result in differences in the level ofthe liquid water surface relative to the structure of the heat transportelement 104 at different positions along the length of the heattransport element 104. As is explained above, the level of the liquidwater surface may be varied. Accordingly, the minor differences in levelcaused by small deviations from the horizontal may be accommodated.

In the illustrated example of the first embodiment, each of the upperand lower sheets 114 and 115 has a dimpled profile. This dimpled profileis shown in more detail in FIGS. 11A and 11B. FIG. 11A shows a plan viewfrom above of a part of the upper sheet 114. FIG. 11B shows a crosssection through the upper sheet 114 along the line D-D in FIG. 11A.

As is shown in FIG. 11A, a plurality of dimples 127 are formed in theflat upper surface 104 a of the heat transport element 104 in the uppersheet 114. The dimples 127 are formed in straight rows and columns toform a regular two dimensional square array, and are spaced apartleaving a flat strip 128 between each row of dimples 127.

Each dimple 127 comprises a looped recess 127 a having a circular innerperimeter 127 b and a square outer perimeter 127 c. The square outerperimeter 127 c has rounded off corners 127 d. Within the circular innerperimeter 127 b a circular region 127 e is raised relative to the loopedrecess 127. The circular region 127 e is at the same level as thesurface 104 a of the flat strips of the upper sheet 115 outside thedimple 127.

The flat strips 128 run transversely across the upper sheet 114 and havethe same width as the width of the first coplanar surfaces 116 a of thecentral sheet 116. The flat strips 128 provide flat areas for bondingwith the first surfaces 116 a of the central sheet 116. The flat strips128 may allow reliable and strong bonds to be made between the firstsurfaces 116 a and the upper sheet 114. The flat strips 128 may allow agood seal to be formed between adjacent upper fluid flow passages 117.

A plurality of dimples 129 are formed in the lower sheet 115. Thedimples 129 are formed in straight rows and columns to form a regulartwo dimensional square array, and are spaced apart leaving a flat strip130 between each row of dimples 129. The dimples 129 in the lower sheet115 are the same as the dimples 127 in the upper sheet 114. The flatstrips 128 run transversely across the upper metal sheet 114 and havethe same width as the width of the first and second coplanar surfaces116 a and 116 b. The flat strips 130 provide flat areas for bonding withthe second surfaces 116 b of the central sheet 116. The flat strips 130may allow reliable and strong bonds to be made between the secondsurfaces 116 b and the lower sheet 115.

In the illustrated example of the first embodiment of the invention boththe dimples 127 in the upper sheet 114 and the dimples 130 in the lowersheet 115 are formed by downward recesses. Accordingly, the dimples 127in the upper sheet 114 have recesses extending into the heat transportelement 104, while the dimples 130 in the lower sheet 115 have recessesextending out of the heat transport element 104. In other examples thedimples 127 and 130 may be formed by recesses extending upwardly, or byrecesses extending in opposite directions.

The array of dimples 130 on the lower metal sheet 115 extends across theflat part of the lower sheet 115, but does not extend into thesemi-cylindrical surface of the outwardly projecting section 110.Further, the array of dimples 130 on the lower sheet 115 has dimplesomitted from the array at the locations of the hollow ridges 125.

The dimples 127 and 130 may increase the rigidity of the upper and lowersheets 114 and 115. This may tend to reduce the amount of flexing of theheat transport element 104 in use. This may prevent damage to thephotovoltaic elements 105 by reducing the amount of mechanical stressapplied to the photovoltaic elements 105. This may allow the upper,lower, and/or central sheets 114, 115, 116, to be thinner, which mayreduce weight and costs. This may allow the upper sheet 114 to bethinner, which may improve the transfer of heat from the photovoltaicelements 105 into the liquid water within the upper fluid flow channels117.

The surfaces of the dimples 127 may provide additional nucleation sitesfor the formation of water vapor bubbles 122, which may improveefficiency.

In examples where adhesive is used to attach the photovoltaic elements105 to the heat transport element 104 the dimples 127 on the flat uppersurface 104 a of the heat transport element 104 may provide reservoirsfor the adhesive. This may allow more secure attachment of thephotovoltaic elements 105. This may allow a thinner layer of adhesive tobe used, which may improve the transfer of heat from the photovoltaicelements 105 into the liquid water within the upper fluid flow channels117.

As discussed above the heat transport element 104 has a flat uppersurface 104 a formed by an upper sheet 114 with a dimpled profile. Inaddition the upper sheet 114 is has two longitudinal recesses 129running across in its upper surface 104 a which form two paralleltroughs running along the upper surface 104 a of the heat transportelement 104. FIG. 12 shows one of these recesses 129. Electricallyconductive ribbons or wires 130 run along the longitudinal recesses 129between the heat transport element 104 and the photovoltaic elements105. The wires 130 are electrically connected to the photovoltaicelements 105 and to the conductors 21 which pass through the cap 12 toprovide a conductive path to carry the electrical power generated by thephotovoltaic elements 105 out of the sealed transparent tube 103. Thiselectrical power may be supplied to an inverter for voltage conversionand/or for conversion to alternating current for supply to a domestic ormains electrical system.

In examples where adhesive is used to attach the photovoltaic elements105 to the heat transport element 104, an electrically insulatingadhesive can be used to electrically insulate the electricallyconductive ribbons or wires 130 from the photovoltaic elements 105 andfrom the upper surface 104 a of the heat transport element 104. Theelectrically insulating adhesive can also be used to electricallyinsulate the photovoltaic elements 105 from the upper surface 104 a ofthe heat transport element 104.

In the first embodiment the longitudinal recesses 129 runperpendicularly to the fluid flow channels 117 and 118. Accordingly,each of the first surfaces 116 a of the central metal sheet 116 has tworecesses to receive the longitudinal recesses 129.

In the illustrated example of the first embodiment each dimple 127comprises a looped recess with a circular inner perimeter 127 b and asquare outer perimeter 127 c, with the circular region 127 e at the samelevel as the surface 104 a of the flat strips of the upper metal sheet115 outside the dimple 127. In some examples the circular region 127 emay not be at the same level as the surface 104 a of the flat strips ofthe upper metal sheet 115 outside the dimple 127. In other examplesdifferent dimple shapes and/or profiles may be used. In some examplesthe perimeters may have different shapes. In some examples the circularregion 127 e may not be at the same level as the surface 104 a of theflat strips of the upper metal sheet 115 outside the dimple 127. In someexamples the dimples may simply comprise a recessed region, rather thana recessed outer region surrounding a relatively raised inner region.

In the illustrated example of the first embodiment 0.2 mm thick tincoated mild steel sheets are used to form the different sheets of theheat transport element. In alternative examples other thicknesses may beused, in particular 0.1 mm thick tin coated mild steel sheets may beused. The use of a thinner upper metal sheet may improve the rate ofheat energy transfer from the photovoltaic elements to the water insidethe upper fluid flow channels. In other examples the different sheetsmay have different thicknesses.

In the illustrated example of the first embodiment the spacing betweenthe upper sheet 114 and the parallel lower sheet 115 is 1.8 mm at thelocations of the longitudinal recesses 129. Accordingly, the thicknessof the fluid flow channels 117 and 118 at the locations of thelongitudinal recesses 129 is 1.6 mm, since the thickness of the centralsheet is 0.2 mm.

The use of mild steel may avoid or reduce problems produced bydifferential thermal expansion of the silicon semiconductor photovoltaicelements 105 and the heat transport element 104 because the coefficientsof thermal expansion of silicon and mild steel are similar.

The sheets used to form the heat transport element may be shaped bypressing.

In other examples different materials may be used, in particular sheetsof other metals or metal alloys, such as copper or brass may be used. Inother examples the upper, lower and/or partition sheets may be formedfrom materials which are not metals. In other examples there may beopenings in the upper sheet allowing the water inside the upper fluidflow channels to directly contact the back surfaces of the photovoltaicelements to maximize heat transfer. In such examples the thickness ormaterial used to form the upper sheet could be selected without havingto take thermal conductivity into account.

In the illustrated example of the first embodiment the tube 119 iscylindrical. In other examples different tube shapes may be used. Insome examples a tube 119 having an elliptical cross section may be used.The use of an elliptical tube may increase the heat exchange area formedby the exterior of the tube 119 where the water vapor condenses, whichmay improve the heat transfer rate between the water vapor and the fluidwithin the tube 119.

In different examples the size of the tube 119 may be varied. However,the tube 119 should not entirely fill the vapor manifold 111. If thereis insufficient free space within the vapor manifold 111 to allow thewater vapor and liquid water to freely circulate this may adverselyaffect the operation of the heat transport element 104.

In the illustrated example of the first embodiment the tube 119 is madefrom copper. In other examples different materials may be used. The tube119 is connected at only one end to the heat transport element 104, withthe other end, the curved section 119 c, being free to move relative tothe heat transport element 104. Accordingly, differential thermalexpansion of the tube 119 and the heat transport element 104 is notgenerally a problem and will generally not need to be taken into accountwhen selecting materials.

In the first embodiment of the invention the roughening of the surfacesof the upper sheet 114 produced by the tin coating may providenucleation sites, increasing the tendency of the liquid water 121 tovaporize and form bubbles 122 of water vapor. In the first embodiment ofthe invention the roughening of the surfaces of the central sheet 116produced by the tin coating may provide nucleation sites, increasing thetendency of the liquid water 121 to vaporize and form bubbles 122 ofwater vapor.

In some examples other coatings may be added to the surfaces of theupper sheet 114 in order to promote or increase nucleation and formationof bubbles of water vapor. In some examples these coatings may be ofmetals, or plastics. In some examples these coatings may be of PTFE.

In the illustrated example of the first embodiment the different sheetsare soldered together. In alternative embodiments different bondingtechniques may be used. In some examples the different sheets may bebonded by techniques including spot welding, roller welding or adhesive.

In the illustrated example of the first embodiment inner faces of theupper and lower sheets 114 and 115, and both faces of the central metalsheet 116, are coated with a solder layer. In the illustrated examplethe solder layers are 2 to 6 microns thick. Other examples may havedifferent thicknesses.

The edges of the upper and lower sheets 114 and 115 are then solderedtogether to form a gas tight seal between them, and to form a gas tightseal between the upper and lower sheets 114 and 115 and the tube 119. Asis explained above, the central metal sheet 116 is not located betweenthe upper and lower metal sheets 114 and 115 at their edges.

The heat transport element 104 is then heated in an oven to asufficiently high temperature to reflow the solder layers on the upper,lower and central sheets 114, 115, 116, and is simultaneously evacuated.

This manufacturing procedure may ensure good solder bonding between thecentral sheet 116 and the upper and lower sheets 114 and 115. Thismanufacturing procedure may allow a better level of vacuum to beachieved within the heat transport element 104 by evacuating the heattransport element 104 at a high temperature when out-gassing by themetal sheets and solder is taking place.

The solder may microscopically roughen the surfaces of the upper andcentral sheets 114 and 116. This may provide nucleation sites,increasing the tendency of the liquid water 121 to vaporize and formbubbles 122 of water vapor.

In other examples, a solder layer is formed on the central sheet 116only on the parts of the central metal sheet which contact the upper orlower sheets 114 and 115. As can be understood from a comparison ofFIGS. 13 and 14 this will be the contact faces of the first and secondsurfaces 116 a and 116 b. Similarly, in some examples a solder layer isformed on the surfaces of the upper sheet 114 and the lower sheet 115only on the parts of the surfaces which will contact one of the othersheets. Reducing the amount of solder used may reduce costs.

In one example the upper sheet 114 only is coated in solder across itsentire surface, while the central sheet and lower sheet 116 and 115 arecoated in solder only on the parts of the surfaces which will contactone of the other sheets. This may allow the solder layer to providenucleation sites on the surface of the upper sheet 114 forming parts ofthe upper fluid flow channels, while reducing the total amount of solderused.

As explained above, in the illustrated example of the first embodimentthe flow of water vapor and liquid water through the heat transportelement 104 tends to keep the cooled upper surface of the heat transportelement 104 at a uniform operating temperature during operation. Thatis, the cooled upper surface of the heat transport element 104 tends tobe kept isothermal. The isothermal nature of the cooled upper surface ofthe heat transport element 104 tends to give rise to isothermal coolingof the photovoltaic elements 105, where hotter parts of the photovoltaicelements 105 tend to be preferentially cooled so that the photovoltaicelements 105 themselves tend to become isothermal.

Such isothermal cooling provides further advantages in addition to thoseprovided by cooling.

Isothermal cooling may provide the advantage that the appearance of hotspots or regions in the photovoltaic elements 105 produced by heating byincident solar radiation can be reduced or eliminated. Such hot spots orregions can reduce the efficiency of the photovoltaic elements 105.

Isothermal cooling may simplify the control and wiring arrangements ofthe photovoltaic elements 105 by reducing or eliminating any requirementfor compensation for differences in the performance of the differentparts of the photovoltaic elements 105 that are at differenttemperatures.

Isothermal cooling tends to reduce, or prevent, the formation of hotspots or regions in the photovoltaic elements 105. As is explainedabove, this may allow the efficiency of the photovoltaic elements 105 tobe improved at a specific temperature. Further, this may reduce theamount of degradation of the photovoltaic elements 105 caused by highertemperatures.

Still further, this may allow the photovoltaic elements 105 to operatewith a given degree of efficiency at a higher temperature than wouldotherwise be the case. This may allow the solar energy collectorassembly 102 including the photovoltaic elements 105 to be operated at ahigher temperature without reducing the efficiency with which thephotovoltaic elements 105 produce electrical energy.

One example of this effect of isothermal cooling is that the generalfigure quoted above for silicon photovoltaic elements that theefficiency of electrical energy generation generally drops by about0.35% to 0.5% for each degree centigrade of temperature increase above25° C. may not apply to silicon photovoltaic elements that areisothermally cooled. Such isothermally cooled silicon photovoltaicelements having hotspots eliminated or reduced may have a higherthreshold temperature at which the efficiency of electrical energygeneration begins to drop and/or may have a reduced rate of reduction inefficiency for each degree centigrade of temperature increase above thethreshold temperature. Further, the temperature at which there is a riskof permanent degradation of the silicon photovoltaic elements may alsobe increased for isothermally cooled silicon photovoltaic elements.Similar effects may be found in photovoltaic elements formed of othersemiconductor materials.

In some examples, one or more layers of heat conductive material may belocated between the upper sheet 114 and the photovoltaic elements 105.Such layers of heat conductive material may increase the rate of heattransfer between the photovoltaic elements 105 and the upper sheet 114,and thus the rate of heat transfer between the photovoltaic elements 105and the liquid within the upper fluid flow channels 117. Such layers ofheat conductive material may also increase the rate of heat transferlaterally across the photovoltaic elements 105.

Accordingly, providing a layer of heat conductive material may increasethe degree of isothermal cooling and further tend to reduce, oreliminate, the formation of hot spots or regions in the photovoltaicelements 105.

The heat transport element may be used in other applications separatelyfrom the rest of the solar energy converter.

In some examples control methods can be used to control the temperatureof the solar energy collector assembly 102. In some examples thetemperature of the solar energy collector assembly 102 may be controlledby changing the rate of removal of heat energy from the solar energycollector assembly 102.

In some examples the rate of removal of heat energy from the solarenergy collector assembly 102 can be controlled by altering the flowrate of the first operating fluid passing through the tube 119 formingthe heat exchanger 107.

In some examples the rate of removal of heat energy from the solarenergy collector assembly 102 can be controlled by altering the vacuumpressure within the tube 103. This may change the rate of convectiveheat loss from the solar energy collector assembly 102 to the tube 103.In general, heat transferred to the tube 103 will be rapidly lost to theoutside environment by convection and/or conduction.

In some examples the rate of removal of heat energy from the solarenergy collector assembly 102 can be controlled by altering the vacuumpressure within the heat transport element 104. In general, the tendencyof the liquid water within the upper fluid flow channel 117 to vaporizeand form bubbles of vapor 122 will increase as the vacuum pressure isreduced, and the tendency of the liquid water within the upper fluidflow channel 117 to vaporize and form bubbles of vapor 122 will decreaseas the vacuum pressure is increased. As is explained above, the densitydriven circulation of water around the upper and lower fluid flowchannels 117 and 118 and the transport of heat energy along the vapormanifold 111 and the tube 119 are both driven by water vapor.Accordingly, altering the tendency of the liquid water to vaporize byaltering the vacuum pressure may allow the rate of removal of heatenergy from the solar energy collector assembly 102, and the rate ofremoval of heat energy from the photovoltaic elements 105 to becontrolled, and so allow the temperature of the solar energy collectorassembly 102 and photovoltaic elements 105 to be controlled.

Further, the temperature at which rolling boiling of the water 121within the upper fluid flow channel 117 commences will tend to increaseas the vacuum pressure is increased, and will tend to decrease as thevacuum pressure is decreased. Accordingly, in examples where the vacuumpressure within the heat transport element 104 is altered thetemperature at which the water 121 within the upper fluid flow channel117 commences rolling boiling can be changed.

As is explained above, the density driven circulation of water aroundthe upper and lower fluid flow channels 117 and 118 becomes particularlyvigorous, and becomes particularly effective as a heat transportmechanism, when the water 121 within the upper fluid flow channel 117enters a rolling boil state. Accordingly, altering the temperature atwhich the water 121 within the upper fluid flow channel 117 commencesrolling boiling by altering the vacuum pressure may allow the rate ofremoval of heat energy from the solar energy collector assembly 102 andphotovoltaic elements 105 to be controlled, and so allow the temperatureof the solar energy collector assembly 102 and photovoltaic elements 105to be controlled.

In some examples the temperature of the solar energy collector assembly102 may be controlled by changing the amount of solar energy incident onthe solar energy collector assembly 102, and so changing the rate ofabsorption of heat energy by the solar energy collector assembly 102.

In some examples the amount of incident solar energy may be controlledby changing the orientation of the solar energy collector assemblyrelative to the direction of the incident solar energy. This can becarried out using a drive mechanism able to rotate the solar energycollector assembly about one or more axes.

In some examples the amount of incident solar energy may be controlledusing adjustable light intercepting or blocking mechanisms in the pathof the incident solar energy. In some examples variable filters,shutters, stops, or the like may be used. In some examples theseadjustable light intercepting or blocking mechanisms may comprisephysical devices. In some examples these adjustable light interceptingor blocking mechanisms may comprise devices having electronicallycontrolled optical characteristics, such as liquid crystals.

In examples where the temperature of the solar energy collector assemblyand/or the photovoltaic elements are to be controlled, a temperaturesensor and a temperature controller may be provided, together with atemperature control mechanism arranged to carry out one, some, or all,of the methods of controlling temperature described above.

The temperature sensor is arranged to measure the temperature of thesolar energy collector assembly and provide this temperature value tothe temperature controller. The temperature controller can then operatethe temperature control mechanism in a suitable manner to control thetemperature of the solar energy collector assembly to the desired value.

Examples where the temperature of the photovoltaic elements is to becontrolled a temperature sensor arranged to measure the temperature of aphotovoltaic element or elements and provide this temperature value tothe temperature controller may be provided. This may be additional to,or instead of, the temperature sensor arranged to measure thetemperature of the solar energy collector assembly. The temperaturecontroller can then operate the temperature control mechanism in asuitable manner to control the temperature of the photovoltaic elementor elements to the desired value.

In some examples the temperature sensor can be provided on the uppersurface of the solar energy collector assembly. In some examples thetemperature sensor can be formed on the same semiconductor wafer as aphotovoltaic element.

Conveniently, the temperature controller may be a suitably programmedgeneral purpose computer.

The illustrated first embodiment is a hybrid solar energy convertercomprising photovoltaic elements and arranged to convert incident solarradiation into outputs of both electrical energy and hot water. In otherexamples the photovoltaic elements may be omitted to provide a solarenergy converter arranged to convert incident solar radiation into anoutput of hot water.

Second Embodiment

In a second embodiment a different arrangement of the tube forming thefirst heat exchanger is used. Otherwise, the second embodiment issubstantially similar to the first embodiment.

FIG. 13 shows a cut away plan view from below of a heat transportelement 104 according to the second embodiment with the outwardlyprojecting section 110 removed, so that the heat exchanger 107 isvisible.

As shown in FIG. 13, in the second embodiment the heat exchanger 107comprises a cylindrical tube 133 which extends within the vapor manifold111 along the entire length of the vapor manifold 111. The cylindricaltube 133 comprises a first straight section 133 a forming the heatexchanger 107 which extends parallel to the sides of the vapor manifold111, such as the upper surface 104 a and the projecting section 110, andextends through one end 108 of the vapor manifold 111, along the entirelength of the vapor manifold 111, and through the other end 108 of thevapor manifold 111. A straight return section 133 b extends behind therear face of the heat transport element 104 through the end cap 120 tothe support element 106.

The first and second sections 133 a and 133 b are connected together bya curved section 133 c.

In use, the first fluid passes along the cylindrical tube 133 asindicated by the arrows in FIG. 13, so that the first fluid passesthrough the first straight section 133 a, the curved section 133 c andthe second straight section 133 b, so that the first fluid passesthrough the cylindrical tube 119, and travels along substantially theentire length of the vapor manifold 111.

In the illustrated example the first and second sections 133 a and 133 bof the cylindrical tube 133 are arranged one above the other and thefirst fluid enters through the first section 133 a and leaves throughthe second section 133 b. In other examples the direction of flow of thefirst fluid may be reversed. In other examples the first and secondsections 133 a and 133 b may be differently arranged.

Similarly to the first embodiment, in other examples the tube 133 may beshapes other than cylindrical. In some examples the tube 133 may have anelliptical cross section. In some examples the cylindrical tube 133 mayhave a different cross sectional shape at different positions. Forexample, the first straight section 133 a may have an elliptical crosssection, while the second straight section 133 b and the curved section133 c may have a circular cross section.

In different examples the size of the tube 133 may be varied. However,the tube section 133 a should not entirely fill the vapor manifold 111.If there is insufficient free space within the vapor manifold 111 toallow the water vapor and liquid water to freely circulate this mayadversely affect the operation of the heat transport element 104.

In the illustrated example the tube 133 is made from mild steel. In someexamples the tube 133 and the heat transport element 104 may be formedof other materials.

In some examples, different materials may be used for the tube 133 andthe heat transport element 104. In such examples, since the tube section133 a is connected at both ends to the heat transport element 104, itmay be necessary to take differential thermal expansion of the tube 133and the heat transport element 104 into account when selectingmaterials. In such examples where different materials are used, the tube133 and/or one or both of the end faces 108 of the vapor manifold 111could be formed with flexible elements to allow relative movement of thetube 133 and the heat transport element 104 in order to accommodatedifferential thermal expansion. Such flexible elements could be formedfrom the material of the tube 133 and/or the end faces 108, or could beseparate components. This may allow a wider range of materials to beused. Such flexible elements could be formed at one or both ends of theheat transport element 104. In one example the flexible element could beprovided by forming a bellows arrangement in the wall of the tube 133.

Third Embodiment

In a third embodiment a different arrangement of the tube forming thefirst heat exchanger is used. Otherwise, the third embodiment issubstantially similar to the first embodiment.

FIG. 14 shows a cut away plan view from below of a heat transportelement 104 according to the third embodiment with the outwardlyprojecting section 110 removed, so that the heat exchanger 107 isvisible.

As can be best seen in FIG. 14, the heat exchanger 107 is comprises acylindrical tube assembly 134 which extends within the vapor manifold111 along substantially the entire length of the vapor manifold 111. Thecylindrical tube assembly 134 comprises an inner concentric tube 134 aand an outer concentric tube 134 b. The tube assembly 134 is shown incross section in FIG. 14, to allow the both of the concentric tubes 134a and 134 b to be seen. The inner and outer concentric tubes 134 a and134 b run parallel to the sides of the vapor manifold 111, such as theupper surface 104 a and the projecting section 110, and extend alongsubstantially the entire length of the vapor manifold 111. The outerconcentric tube 134 b extends slightly further than the inner concentrictube 134 a, the outer concentric tube 134 b is sealed by an end cap 134c, and the inner concentric tube 134 a is open ended, so that the innerand outer concentric tubes 134 a and 134 b form a single fluid flowpath.

In use, the first fluid passes through the cylindrical tube assembly 134as indicated by the arrows in FIG. 14, so that the first fluid passesthrough the inner concentric tube 134 a along substantially the entirelength of the vapor manifold 111, then moves outwardly into the outerconcentric tube 134 b, and travels through an annular channel 134 ddefined between the inner concentric tube 134 a and the outer concentrictube 134 b back along substantially the entire length of the vapormanifold 111.

The water vapor in the vacuum within the vapor manifold 111 condenses onthe outer surface of the outer concentric tube 134 b and transfers heatenergy from the heat transfer element 104 to the first fluid within thetube assembly 134.

In the illustrated example the first fluid enters through the innerconcentric tube 134 a and leaves through the annular channel 134 d. Inother examples the direction of flow of the first fluid may be reversed.

In the illustrated example of the first embodiment the tube 119 iscylindrical. In other examples different tube shapes may be used. Insome examples a tube 119 having an elliptical cross section may be used.The use of an elliptical tube may increase the heat exchange area formedby the exterior of the tube 119 where the water vapor condenses, whichmay improve the heat transfer rate between the water vapor and the fluidwithin the tube 119.

In different examples the size of the tube assembly 134 may be varied.However, the tube assembly 134 should not entirely fill the vapormanifold 111.

In the illustrated example of the third embodiment the tube assembly 134is made from copper. In other examples different materials may be used.The tube assembly 134 is connected at only one end to the heat transportelement 104, with the other end being free to move relative to the heattransport element 104. Accordingly, differential thermal expansion ofthe tube assembly 134 and the heat transport element 104 is notgenerally a problem and will generally not need to be considered whenselecting materials.

Fourth Embodiment

In a fourth embodiment a different arrangement of the tube forming thefirst heat exchanger is used. Otherwise, the fourth embodiment issubstantially similar to the first embodiment. The fourth embodiment maybe used in examples where the tube 103 has an end cap 120 at each end.

FIG. 15 shows a cut away plan view from below of a heat transportelement 104 according to the fourth embodiment with the outwardlyprojecting section 110 removed, so that the heat exchanger 107 isvisible.

As shown in FIG. 15, in the fourth embodiment the heat exchanger 107comprises a cylindrical tube 135 which extends within the vapor manifold111 along the entire length of the vapor manifold 111. The cylindricaltube 135 comprises a straight section 135 a forming the heat exchanger107 which extends parallel to the sides of the vapor manifold 111, suchas the upper surface 104 a and the projecting section 110, and extendsthrough one end 108 of the vapor manifold 111, along the entire lengthof the vapor manifold 111, and through the other end 108 of the vapormanifold 111.

At each end of the straight section 135 a a respective connectingsection 135 b of the tube 135 extends through the end cap 120 at eachend of the tube 103 to a support structure, such as the support element106.

In use, the first fluid passes along the cylindrical tube 135 asindicated by the arrows in FIG. 14, so that the first fluid passesthrough the first straight section 135 a and travels along substantiallythe entire length of the vapor manifold 111.

Similarly to the first embodiment, in other examples the tube 135 may beshapes other than cylindrical. In some examples the tube 135 may have anelliptical cross section. In some examples the cylindrical tube 135 mayhave a different cross sectional shape at different positions. Forexample, the first straight section 135 a may have an elliptical crosssection, while the connecting sections 135 b of the tube 135 have acircular cross section.

In different examples the size of the tube 135 may be varied. However,the tube section 135 a should not entirely fill the vapor manifold 111.If there is insufficient free space within the vapor manifold 111 toallow the water vapor and liquid water to freely circulate this mayadversely affect the operation of the heat transport element 104.

In the illustrated example the tube 135 is made from mild steel. In someexamples the tube 135 and the heat transport element 104 may be formedof other materials.

In some examples, different materials may be used for the tube 135 andthe heat transport element 104. In such examples, since the tube section135 a is connected at both ends to the heat transport element 104, itmay be necessary to take differential thermal expansion of the tube 135and the heat transport element 104 into account when selectingmaterials. In such examples where different materials are used, the tube135 and/or one or both of the end faces 108 of the vapor manifold 111could be formed with flexible elements to allow relative movement of thetube 135 and the heat transport element 104 in order to accommodatedifferential thermal expansion. Such flexible elements could be formedfrom the material of the tube 135 and/or the end faces 108, or could beseparate components. This may allow a wider range of materials to beused. Such flexible elements could be formed at one or both ends of theheat transport element 104. In one example the flexible element could beprovided by forming a bellows arrangement in the wall of the tube 135.

As shown in FIG. 15, the connecting sections 135 b of the tube 135 arebent through bends or elbows. These bends may provide flexibility or“give” in order to accommodate differential thermal expansion of theheat transport element 104 and tube 135 and the tube 103. In otherexamples different means to accommodate differential thermal expansionmay be provided.

Fifth Embodiment

Apparatus according to a fifth embodiment of the present invention isillustrated in FIG. 16. FIG. 16 shows a general exterior view of a fifthembodiment of a hybrid solar energy converter 201 according to thepresent invention.

Overview

In the fifth embodiment, the hybrid solar energy converter 201 includesa solar energy collector assembly 202 housed within a sealed transparenttube 203. The solar energy collector assembly 202 includes a heattransport element 204 and an array of photovoltaic elements 205 mountedon a front surface of the heat transport element 204, the front surfacebeing the surface exposed to incident solar radiation in use. The hybridsolar energy converter 201 also includes a support assembly 206 at oneend of the transparent tube 203. One end of the solar energy collectorassembly 202 is connected to the support assembly 206. Similarly to thefirst embodiment, in different examples the photovoltaic elements 205may be formed of silicon, or gallium arsenide, or other suitablesemiconductor materials. In other examples organic photovoltaic elementsmay be used. In other examples hybrid photovoltaic elements may be used.

In the fifth embodiment, the support assembly 206 supplies a first fluidto a heat exchanger 207 arranged to transfer heat energy from the heattransport element 204 to the first fluid.

In one possible example, in use the hybrid solar energy converter 201may be mounted on a wall. Accordingly, suitable mounting brackets may beprovided.

In overview, the operation of the hybrid solar energy converter 201 ofthe fifth embodiment is similar to operation of the hybrid solar energyconverter 101 of the first embodiment. Solar energy incident on thehybrid solar energy converter 201 passes through the sealed transparenttube 203 and is incident on the photovoltaic elements 205 of the solarenergy collector assembly 202. The photovoltaic elements 205 convert apart of the energy of the incident solar energy into electrical energy,and convert a part of the energy of the incident solar energy into heatenergy. A further part of the incident solar energy may be incident onany parts of the solar energy collector assembly 202 which are notcovered by the photovoltaic elements 205, and this further part of theincident solar energy may also be converted into heat energy.

In general, it is desirable to maximize the proportion of the surface ofthe solar energy collector assembly 202 exposed to incident solar energywhich is covered by the photovoltaic elements 205, and to minimize theproportion which is not so covered. However, in some circumstances itmay be preferred to leave some parts of this exposed surface uncovered,for example to simplify manufacture and/or assembly of the solar energycollector assembly 202 and attachment of the photovoltaic elements 205to the solar energy collector assembly 202.

The electrical energy produced by the photovoltaic elements 205 iscarried along the heat transport element 204 by electrical conductorsand away from the solar energy converter 201 for use. The heat energyabsorbed by the photovoltaic elements 205 is transferred into the heattransport element 204, cooling the photovoltaic elements 205, and thentransferred into the first fluid by the heat exchanger 207.

In one typical arrangement, the hybrid solar energy converter 201 may beused to generate electricity, and to generate hot water. Similarly tothe first embodiment, in this arrangement the first fluid is water andthe heat energy transferred to the heat exchanger 207 is transferredinto a pumped water supply flowing through the heat exchanger 207 toheat the water. This heated water is then used by a domestic orindustrial hot water system, and the electrical energy produced by thephotovoltaic elements 205 is supplied to an electrical supply system.

Transparent Tube

In the fifth embodiment illustrated in FIG. 16 the sealed transparenttube 203 is similar to the sealed transparent tube 103 of the firstembodiment, having one closed domed end and one open end sealed by anend cap 220. The interior of the tube 203 is at least partiallyevacuated. That is, the interior of the tube 203 is below normalatmospheric pressure.

The pressure of the vacuum within the tube 203 may be 10⁻³ mbar. Otherpressures may be used, as discussed regarding the first embodiment. Insome examples the vacuum pressure may be in the range 10⁻² mbar to 10⁻⁶mbar. In general, it is expected that lower vacuum pressure, or in otherwords a harder vacuum, will provide greater insulating benefits.Further, it is expected that lower vacuum pressure, or in other words aharder vacuum, will provide greater protection from environmental damagein examples where the photovoltaic elements are not encapsulated. Inpractice the benefits of using a lower vacuum pressure may need to bebalanced against the increased cost of achieving a lower vacuumpressure. In some examples a vacuum pressure of 10⁻² mbar, or lower, maybe used.

In an alternative example the sealed transparent tube 203 may be filledwith an inert gas instead of being evacuated. In particular, the inertgas may be nitrogen.

In another alternative example the sealed transparent tube 203 may befilled with an inert gas at a reduced pressure. In some examples thismay be achieved by filling the tube 203 with the inert gas and thenevacuating the tube 203. In particular, the inert gas may be nitrogen.

In the illustrated fifth embodiment the tube 203 is cylindrical having acircular cross section. Similarly to the first and second embodiments,in alternative examples the tube 203 may have other shapes. In someexamples the cross sectional size and/or shape of the tube 203 may varyat different positions along its length. In an alternative example thetube 203 may have an elliptical cross section. In particular, the tube203 may have an elliptical cross section with the long axis of theellipse aligned with the plane of the solar energy collector assembly202.

In the illustrated fifth embodiment the tube 203 is formed of glass. Inalternative examples suitable transparent plastics materials orlaminated structures may be used to form the tube 203.

In the illustrated fifth embodiment the tube 203 is transparent. Inalternative examples the tube may be only partially transparent.

In the illustrated fifth embodiment the metal end cap 220 may be bondedto the glass tube 203 by adhesive. In other embodiments alternativeglass to metal bonding techniques may be used, for example welding,brazing or soldering.

Similarly to the first embodiment the tube 203 has a metal end cap 220at one end. In alternative examples the end cap 220 may be made of othermaterials. In some examples the end cap 220 may be made of glass. Thismay reduce conductive heat losses from the collector assembly 202.

Collector Assembly

In the fifth embodiment, the solar energy collector assembly 202includes a heat transport element 204 and an array of photovoltaicelements 205 mounted on one surface of the heat transport element 204.In order to allow radiant solar energy to be incident on thephotovoltaic elements 205 the array of photovoltaic elements 205 aremounted on the surface of the heat transport element 204 which isexposed to the incident radiant solar energy in operation of the hybridsolar energy converter 201. In the fifth embodiment the heat transportelement 204 may be mounted vertically. In examples where the heattransport element 204 is not mounted vertically the surface which isexposed to the incident radiant solar energy in operation will usuallybe the upper surface of the heat transport element 204.

In some arrangements the surface of the heat transport element 204exposed to the incident radiant solar energy may not be the uppersurface. In particular, this would be the case if the incident solarradiant energy was incident horizontally or from below, for exampleafter redirection by an optical system such as a mirror.

In the illustrated example of the fifth embodiment, the solar energycollector assembly 202 is supported by cylindrical tubes 234 of the heattransport element 204. The cylindrical tubes 234 pass through the endcap 220 and into the support assembly 206, as will be explained in moredetail below. Where the cylindrical tube 234 passes through the end cap220 the cylindrical tube 234 is soldered to the end cap 220 to retainthe cylindrical tube 234 in place and support the solar energy collectorassembly 102.

The cylindrical tubes 234 are assemblies comprising inner and outerconcentric tubes 234 a and 234 b, similarly to the heat exchangerarrangement according to the third embodiment. In alternative examplesthe heat exchanger arrangements according to the first, second or fourthembodiments may be used instead.

In alternative examples the cylindrical tubes 234 may be secured to theend cap 220 in other ways. In one example the cylindrical tubes 234 maybe welded to the end cap 220.

The supporting of the solar energy collector assembly 202 by physicalconnections through the cylindrical tubes 234 may increase theefficiency with which heat can be collected from incident solar energyby the solar energy collector assembly 202. Having the solar energycollector assembly 202 supported by physical connections only throughthe cylindrical tubes 234 may reduce conductive heat loss from the solarenergy collector assembly 202 into the supporting structure outside thetransparent tube.

In the illustrated example of the fifth embodiment the heat transportelement 204 has a substantially flat front surface 204 a. Each of thephotovoltaic elements 205 is square, and the width of the heat transportelement 204 is the same as the width of each square photovoltaic element205. Six square photovoltaic elements 105 are mounted side by side toone another along the length of the heat transport element 204.Substantially the entire front face of the heat transport element 204 iscovered by the photovoltaic elements 205. Covering a large proportion ofthe upper surface 204 a of the heat transport element 204 withphotovoltaic elements 205 may increase the efficiency of the hybridsolar energy converter 201.

In one example the square photovoltaic elements 205 may each be a 125 mmby 125 mm square and 0.2 mm thick. In another example the squarephotovoltaic elements may each be a 156 mm by 156 mm square. In otherexamples, photovoltaic elements having other sizes or shapes may beused.

The photovoltaic elements 205 are bonded to the substantially flat uppersurface 204 a of the heat transport element 204 using a layer of heatconducting adhesive in a similar manner to the first embodiment. Theadhesive bonding layer is electrically insulating. The adhesive bondinglayer between the photovoltaic elements 205 and the heat transportelement 204 is arranged to be thin. This may improve the degree ofthermal conduction between the photovoltaic elements 205 and the heattransport element 204. This may increase the rate of heat transferlaterally across the photovoltaic elements 205. An adhesive materialloaded with solid spheres of a predetermined size may be used to formthe adhesive bonding layer. This may allow a thin adhesive layer to beconsistently and reliably formed. The adhesive bonding layer is formedof a flexible or “forgiving” adhesive material. This may relievestresses in the assembled solar energy collector assembly 202 and reduceany stress applied to the photovoltaic elements 205.

The photovoltaic elements 205 are semiconductor photovoltaic elementsformed of silicon. In one embodiment the photovoltaic elements areformed of single-crystal silicon. In one embodiment the photovoltaicelements are formed of amorphous silicon. In one embodiment thephotovoltaic elements are formed of polycrystalline silicon, orpolysilicon. In other embodiments alternative types of semiconductorphotovoltaic elements may be used.

Similarly to the first embodiments, in operation of the hybrid solarenergy converter 201 the photovoltaic elements 205 are cooled by theheat transport element 204, which may provide similar advantages tothose discussed above. This cooling may allow the temperature of thephotovoltaic elements 205 to be maintained at a desired value.

This cooling may provide the advantage that the appearance of hot spotsor regions in the photovoltaic elements 205 can be reduced oreliminated, and the temperature of the photovoltaic elements 205maintained at a uniform desired value. Such hot spots or regions may forexample be produced by heating by incident solar radiation, byinhomogeneities or faults in the photovoltaic elements 205, or by acombination of, or interaction between, these causes.

As discussed above regarding the first embodiment, such hot spots orregions can reduce the efficiency of the photovoltaic elements 205 inthe short term, and may also degrade the performance of the photovoltaicelements 205 in the longer term.

Accordingly, maintaining the photovoltaic elements 205 at a more uniformtemperature value and reducing, or eliminating, hot spots or regions mayimprove the efficiency of the photovoltaic elements 205 at a specifictemperature, and may reduce the amount of degradation of thephotovoltaic elements 205 caused by higher temperatures.

This may allow the photovoltaic elements 205 to operate at a higheroverall temperature than would otherwise be the case, for the samereasons as discussed regarding the first embodiment.

The illustrated example of the fifth embodiment has a solar energycollector assembly 202 supported only by physical connections throughthe cylindrical tubes 234. In other examples alternative supportingarrangements may be used. In some examples the solar energy collectorassembly 202 may be supported by a physical connections both ends of thesolar energy collector assembly 202. In some examples, the physicalconnections at one end of the solar energy collector assembly may be thethrough the cylindrical tubes 234. In general, it is advantageous tominimize the number of physical supports in order to minimize the escapeof heat from the solar energy collector assembly by conduction throughthe physical supports.

In other examples the number of photovoltaic elements 205 mounted on theheat transport element 204 may be different. In other examples therelative sizes of the photovoltaic elements 205 and the heat transportelement 204 may be different.

In some examples the adhesive layer may comprise an epoxy resin whichremains non-brittle after curing.

In other examples the adhesive layer may be formed by a double sidedadhesive tape.

Heat Transport Element

The heat transport element 204 according to the fifth embodiment isshown in more detail in a cut away view in FIG. 17.

In the fifth embodiment, the heat transport element 204 is generallyrectangular. The heat transport element 204 has a flat front surface 204a and a rear surface 204 b which is flat across most of its area, andhas three outwardly projecting sections 210 spaced out along its length,with a first outwardly projecting section 210 at an upper end of theheat transport element 204, a second outwardly projecting section 210located one third of the way along the length of the heat transportelement 204, and a third outwardly projecting section 210 located twothirds of the way along the length of the heat transport element 204.

The heat transport element 204 is divided into three sections, an uppersection 204 c, a central section 204 d, and a lower section 204 e. Eachsection 204 c to 204 e is cooled by a separate density drivencirculation acting as a heat transport mechanism similar to themechanism of the second embodiment and comprising a respective one ofthe three outwardly projecting sections 210. Each of the three sections204 c to 204 e supports and cools two of the six photovoltaic elements205.

Each outwardly projecting section 210 contains and defines an elongateheat transfer chamber or vapor manifold 211 extending substantially fromone side to the other of the heat transport element 204. In operationthe heat transport element 204 is arranged to be longitudinally sloping,so that the heat transport element 204 has an upper end and a lower end.The heat transport element 204 may be arranged longitudinallyvertically, or at an angle to the vertical.

The heat transport element 204 has a front surface 204 a formed by afront sheet 214 and a rear surface 204 b formed by a rear sheet 215.Three central sheets 216 are located between the front sheet 214 and therear sheet 215, with one of the central sheets 216 in each of thesections 204 a to 204 c, so that fluid flow passages 217 and 218 runninglongitudinally along the heat transport element 204 are defined betweeneach central sheet 216 and each of the front sheet 214 and the rearsheet 215. Since the heat transport element 204 is longitudinallysloping the fluid flow passages 217 and 218 running longitudinally alongthe heat transport element 204 will be sloped along their lengths.

Each central sheet 216 has a similar profile to the central sheet 116 ofthe first embodiment, except that, compared to the second embodiment,the profile of the central sheets 216 of the third embodiment is rotatedthrough 90° to define flow channels running longitudinally along theheat transport element 204. The cross-sectional profile of thecorrugated central sheets 216 can be understood as a zig-zag profilewith the points of the zig-zag forming the peaks and troughs beingflattened.

To be more specific, in the illustrated example of the fifth embodimentthe central sheets 216 each comprise a plurality of flat surfacesconnected by folds running longitudinally along the heat transportelement 204. Accordingly, the front, rear, and central sheets 214, 215,216 define a plurality of trapezoid cross-section front fluid flowchannels 217 and rear fluid flow channels 218 between them. The frontfluid flow channels 217 are defined between the front sheet 214 and thecentral sheets 216. The rear fluid flow channels 218 are defined betweenthe rear sheet 215 and the central sheets 216. The trapezoid front fluidflow channels 271 are arranged so that the larger one of the twoparallel faces of each trapezoid channel 217 is formed by the uppersheet 214.

The front and rear fluid flow channels 217 and 218 of the fifthembodiment respectively correspond in function to the upper and lowerfluid flow channels 117 and 118 of the second embodiment.

The edges of the heat transport element 204 are formed by bent parts ofthe rear sheet 215, which are bonded to the front sheet 214. Thephotovoltaic elements 205 are bonded to the front sheet 214. At theedges of the heat transport element 204, the front sheet 214 is bondeddirectly to the rear sheet 215, the central sheets 216 are not locatedbetween the front and rear sheets 214 and 215 at their edges.

In some examples the central sheets 216 may extend at least partiallybetween the front and rear sheets 214 and 215 at the side edges of theheat transport element 204 so that the front and rear sheets 214 and 215are both bonded to the central sheets 216. This may assist in locatingand securing the central sheets 216 relative to the front and rearsheets 214 and 215.

As discussed above, the heat transport element 204 has three outwardlyprojecting sections 210 each running transversely across the rearsurface 204 b of the heat transport element 204. Each outwardlyprojecting section 210 is substantially semi-cylindrical and is formedby an outwardly projecting part of the rear sheet 215. Each outwardlyprojecting section 210 defines a vapor manifold 211. The fluid flowchannels 217 and 218 connect to the vapor manifolds 211. It should benoted that the central sheets 216 extend across most of the width of thevapor manifolds 211. Accordingly, the front fluid flow channels 217defined between the front sheet 214 and the central sheets 216 connectto the vapor manifolds 211 towards the top of each vapor manifold 211,while the rear fluid flow channels 218 defined between the rear sheet215 and the central sheets 216 connect to the vapor manifolds 211towards the bottom of each vapor manifold 211.

The front and rear fluid flow channels 217 and 218 are formed into threegroups with the front and rear fluid flow channels 217 and 218 of eachgroup interconnected by one of the vapor manifolds 211. Each group offluid flow channels 217 and 218 extends along one of the sections 204 cto 204 e of the heat transport element 204 and, together with the vapormanifold with which they are connected, forms a separate heat transportmechanism cooling the respective section 204 c to 204 e of the heattransport element 204.

FIG. 17 is an explanatory diagram showing a longitudinal cross sectionof a part of the heat transport element 204 along the line D-D in FIG.16. FIG. 17 shows the section of the heat transport element 204 aroundthe boundary between the central section 204 d and the lower section 204e. The boundary between the central section 204 d and the upper section204 c is identical.

At the top of the lower section 204 e of the heat transport element 204,at the top of the outwardly projecting section 110, there is a wall 231extending transversely across the interior of the heat transport element204. The wall 231 contacts and is bonded to the front and rear sheets214 and 215 and forms a fluid tight seal between the fluid flow channels217 and 218 of the central section 204 d of the heat transport element204 and the vapor manifold 211 of the lower section 204 e of the heattransport element 204. The walls 131 divide the interior of the heattransport element 204 into three separate fluid circulation regionscorresponding to the sections 204 c to 204 e of the heat transportelement 204.

There is a gap 223 between the edge of the central sheet 216 of thecentral section 204 d of the heat transport element 204 and the wall231. This gap 223 allows water to flow between different ones of thefluid flow channels 217 and 218. The gap 223 extends along the side wall231, and forms a fluid manifold 224 interconnecting all of the front andrear fluid flow channels 217 and 218 of the central section 204 d.

In the fourth embodiment the heat exchanger 207 comprises a cylindricaltube assembly 234 which extends within the elongate heat transferchamber or vapor manifold 211 along substantially the entire length ofthe elongate heat transfer chamber or vapor manifold 211. Thecylindrical tube assembly 234 comprises an inner concentric tube 234 aand an outer concentric tube 234 b. The inner and outer concentric tubes234 a and 234 b run parallel to the sides of the vapor manifold 211,such as the upper surface 204 a and the projecting section 210, andextend along substantially the entire length of the vapor manifold 211.The outer concentric tube 234 b extends slightly further than the innerconcentric tube 234 a, the outer concentric tube 234 b is sealed by anend cap, and the inner concentric tube 234 a is open ended, so that theinner and outer concentric tubes 234 a and 234 b form a single fluidflow path.

In use, the first fluid passes through the cylindrical tube assembly234, so that the first fluid passes through the inner concentric tube234 a along substantially the entire length of the vapor manifold 211,then moves outwardly into the outer concentric tube 234 b, and travelsthrough an annular channel 234 d defined between the inner concentrictube 234 a and the outer concentric tube 234 b back along substantiallythe entire length of the vapor manifold 211.

At each edge of the heat transport element 204 the substantiallysemi-cylindrical outwardly projecting section 210 extending most of thewidth of the heat transport element 204 is closed by an end wall 208.The upper and lower sheets 214 and 215 are sealed by the end walls 208so that the interior of the heat transport element 204 is sealed. Thecylindrical tube 234 passes through the end wall 208 and the end cap 220and into the support assembly 206. Each cylindrical tube 234 within thevapor manifold 211 forms a heat exchanger 207 and acts to carry heatenergy from part of the heat transport element 204 away from the hybridsolar energy converter 201, as will be explained below.

The cylindrical tubes 234 physically support the solar energy collectorassembly 202 within the sealed transparent tube 203. There is no otherphysical support of the solar energy collector assembly 202. As in theprevious embodiments this may reduce conductive heat losses from thesolar energy collector assembly 202, which may increase the amount ofuseful heat energy produced by the hybrid solar energy converter 201.

The fluid flow channels 217 and 218 are at least partially filled withdegassed distilled water 221 as a working fluid and the interior of theheat transport element 204 including the fluid flow channels 217 and 218and the vapor manifolds 211 are at least partially evacuated. That isthe interior of the heat transport element 204 is below normalatmospheric pressure. The interior of the heat transport element 104 maybe under a vacuum at a pressure of 10⁻³ mbar.

In the fifth embodiment the amount of water 221 in the fluid flowchannels 217 and 218 is similar to the first embodiment except that theinterior of each of the sections 204 c to 204 e is sealed off from theothers so that the level of the water 221 is independent in each of thesections 204 c to 204 e of the heat transfer element 204.

In each of the three sections 204 c to 204 e the level of the water 221in the fluid flow channels 217 and 218 is such that the upper surface ofthe water 221 in the rear fluid flow channels 218 is level with the endsof the rear fluid flow channels 218 where they connect to the vapormanifold 211. In the illustrated fifth embodiment the level of thesurface of the water 221 in the front fluid flow channels 217 and rearfluid flow channels 218 is the same. Accordingly, in the illustratedfifth embodiment the rear fluid flow channels 218 are filled with liquidwater, while the front fluid flow channels 217 are only partially filledwith liquid water.

Similarly to the first embodiment, in other examples the level of thewater 221 may be different. In some examples the upper surface of thewater 221 in the rear fluid flow channels 218 may be below the vapormanifold 211. In some examples the upper surface of the water 221 in therear fluid flow channels 218 may be above the bottom of the vapormanifold 211, with some water being present in the bottom of the vapormanifold 211.

It is expected that in practice the heat transport element 204 willoperate most efficiently with the upper surface of the water being at,or close to, the point where the lower fluid flow channels 218 contactthe vapor manifold 211. If the level of the water in the heat transportelement 204 is too high, so that the upper surface of the water is toohigh within the vapor manifold 211, the efficiency of operation of theheat transport element 204 may be reduced, for the same reasons as arediscussed regarding the first embodiment.

The upper surface of the water 221 in the front fluid flow channels 217may be higher than in the rear fluid flow channels 218 as a result ofcapillary action. The extent of this capillary effect in any specificexample will depend upon the dimensions of the front fluid flow channels217. In the illustrated fifth embodiment some of the inner surface ofthe upper sheet 214, that is, the surface forming a part of the upperfluid flow channels 217, is above the surface of the water 221. In someexamples the front fluid flow channels 217 may have a small enoughcross-sectional area that the upper surface of the water 221 in thefront fluid flow channels 217 is at the ends of the front fluid flowchannels 217 due to capillary action.

Similarly to the first embodiment, it is not necessary that the innersurface of the front sheet 214, that is, the surface forming a part ofthe front fluid flow channels 217, is below the surface of the water 221at a position corresponding to the location of the uppermost parts ofthe photovoltaic elements 205 for each of the sections 204 c to 204 e ofthe heat transport element 204. However, in some embodiments this may bethe case.

In operation of the fifth embodiment, when the solar energy collectorassembly 202 is exposed to incident solar radiative energy, thephotovoltaic elements 205 absorb some of this energy, converting a partof the absorbed energy into electrical energy. The remainder of theabsorbed energy is converted into heat energy, raising the temperatureof the photovoltaic elements 205. The absorbed heat energy flows fromthe photovoltaic elements 205 into the heat transport element 204, beingtransmitted through the front sheet 214 and into the water 221 insidethe front fluid flow channels 217, which water is in contact with theinner surface of the front metal sheet 214 across the larger parallelfaces of the trapezoid front fluid flow channels 217.

The liquid water 221 inside the front fluid flow channels 217 absorbsthe heat energy from the photovoltaic elements 205 passing through thefront sheet 214 and vaporizes, producing bubbles 222 of steam or watervapor. At the vacuum pressure of 10⁻³ mbar inside the front fluid flowchannels 217 water boils from around 0° C., so that the water 221vaporizes readily at the normal operating temperatures of the hybridsolar energy converter 201.

As discussed above regarding the first embodiment, the bubbles 222 ofwater vapor are less dense than the liquid water 221. Further, asexplained above the front fluid flow channels 117 are sloping alongtheir lengths. Accordingly, as a result of this density difference thewater vapor bubbles 222 travel upwards along the front fluid flowchannels 217 towards the top of the heat transport element 204 and thesurface of the water 221. When a bubble of water vapor 222 reaches thesurface of the water 221 the vapor is released into the vacuum above thewater 221 in the respective vapor manifold 211. Further, the bubbles 222will give rise to pistonic driving in a similar manner to the firstembodiment. In the illustrated fifth embodiment, where some of the innersurface of the upper sheet 214 is above the surface of the water 221,this pumping of liquid water upwards along the upper flow channels 217ensures that the part of the inner surface of the upper sheet 214 abovethe surface of the water 221 is in contact with a flow of water so thatit can be cooled.

The bursting of the bubbles of water vapor at the water surface and anypistonic pumping of liquid water out of the ends of the front fluid flowchannels 217 may generate droplets of liquid water, and may project atleast some of these water droplets into the vacuum within the respectivevapor manifold 211 above the water surface. As a result, the heattransfer mechanism may be a multi-phase system comprising liquid water,water vapor and droplets of liquid water, and not just a two-phasesystem comprising liquid water and water vapor only. The presence ofsuch droplets of water in the vacuum, and any pumping of liquid waterout of the ends of the front fluid flow channels 217, may enhance therate of vaporization by increasing the surface area of the water exposedto the vacuum.

Similarly to the first embodiment, the water vapor in the vacuum withinthe heat transfer chamber or vapor manifold 211 travels at a very highspeed through the vacuum within the vapor manifold 211 into contact withthe exterior surface of the outer tube 234 b forming the heat exchanger207. The travel speed of the hot water vapor in the vacuum is very fast,approximating to the thermal speed of the water vapor molecules. Thewater vapor condenses on the external surface of the outer tube 234 b,which acts as a heat exchange surface. The condensed water falls off theouter tube 234 b to the bottom of the vapor manifold 211, and isreturned back into the water 221 within the lower fluid flow channels218. This generating of hot water vapor within the upper fluid flowchannels 217 and the vapor manifold 211, and the condensing of the hotwater vapor on the outer tube 234 b, followed by return of the condensedwater into the fluid flow channels 217 and 218, transfers heat energyfrom the heat transfer element 204 to the first fluid within the tube234.

Thus, the working fluid is caused to circulate on a working fluidpathway inside the heat transfer device such that the heat of theworking fluid is transferred to the heat exchanger in the heat transferchamber. In the illustrated example the working fluid pathway is formedby the fluid flow channels 217 and 218 and the heat of the working fluidis absorbed from the cooled surface 204 a of the heat transport element204.

The first fluid flows from the support element 206, through the heattransfer element 204, where it absorbs heat, and the heated fluid thenreturns to the support element 206. In the illustrated second embodimentthe first fluid is water and a pumped water supply passes through thesupport element 206 and the cylindrical tube 234.

Any liquid water ejected from the front fluid flow channels 217 into avapor manifold 211 which does not vaporize will also fall to the bottomof the respective vapor manifold 211, and is returned back into thewater 221 within the rear fluid flow channels 218 associated with thatvapor manifold 211.

The description indicates that water droplets and condensed water fallsinto the bottom of the vapor manifold 211. In some examples, dependingupon the orientation of the collector assembly 202, some or all of thiswater may fall into the lower fluid flow channels 218 withoutnecessarily contacting the surface of the vapor manifold 211.

The location of the heat exchangers 207 within the heat transferchambers or vapor manifolds 211 may improve the efficiency of the heattransfer element 204 by providing a short path for water vapor to travelbetween the upper surface of the working fluid and the heat exchanger207.

As is explained above all of the front and rear fluid flow channels 217and 218 in each section 204 c to 204 e of the heat transfer element 204are interconnected by the respective fluid manifold 224 formed by therespective gap 223. Accordingly, within each section 204 c to 204 e ofthe heat transfer element 204, it is not important which of the rearfluid flow channels 218 is entered by any liquid water returning fromthe respective vapor manifold 211.

As is clear from the description above, each heat transfer chamber orvapor manifold 211 generally includes liquid water in addition to watervapor when the hybrid solar energy converter 201 is operating. However,as is also discussed above, if the level of the water in a section 204 cto 204 e of the heat transport element 204 is too high, so that theupper surface of the water is too high within the respective vapormanifold 211, the efficiency of operation of the heat transport element204 may be reduced. This reduction in efficiency of operation may occurbecause there is insufficient space within the vapor manifold 211 abovethe surface of the water for the movement and evaporation of thedroplets of liquid water. This reduction in efficiency of operation mayoccur because the droplets of liquid water and waves and splashingupwardly of the liquid water surface may reduce the open, or water free,cross sectional area of the vapor manifold at some locations to arelatively small amount, or even to zero, momentarily closing the vapormanifold. This reduction in the open, or water free, cross sectionalarea of the vapor manifold may interfere with the movement of the watervapor in the vacuum within the vapor manifold 211.

In a similar manner to the first embodiment the bubbles 222 of watervapor will tend to move upwardly through the liquid water in the frontfluid flow channel 217 because of the lower density of the water vaporcompared to the liquid water 221, which will result in an upwardbuoyancy force on each bubble 222. Further, the movement of the bubbles222 of water vapor will tend to drive the liquid water 221 in the frontfluid flow channel 217 upwardly, particularly in examples where pistonicdriving takes place. As a result, the bubbles 222 of water vapor causethe water 221 in the front and rear fluid flow channels 217 and 218 ineach section 204 c to 204 e to circulate, with relatively hot liquidwater and bubbles 222 of water vapor flowing upwards along the frontfluid flow channels 217, and relatively cool liquid water flowingdownwards along the rear fluid flow channels 218. The front and rearfluid flow channels 217 and 218 are interconnected by the vapor manifold211 and the fluid manifold 224, as explained above. Accordingly, therelatively hot liquid water flowing upwards along the front fluid flowchannels is continuously replaced by relatively cool liquid water fromthe rear fluid flow channels 218. This circulation is driven primarilyby the difference in density between the water vapor and the liquidwater. However, this circulation may also be driven by convection as aresult of the difference in density between the relatively hot liquidwater in the front fluid flow channels 217 and the relatively coolliquid water in the rear fluid flow channels 218, in a similar manner toa thermosiphon. Accordingly, the front fluid flow channels 217 may beregarded as riser channels, while the rear fluid flow channels 218 maybe regarded as sinker channels or return channels.

As the bubbles 222 of water vapor travel upwardly along the front fluidflow channels 217 the pressure head acting on the bubbles 222 decreases,so that the bubbles 222 tend to expand. As a result, the tendency of thevapor bubbles 222 to collapse and implode is reduced by the effects ofthe expansion and decreasing pressure as the bubbles 222 move upwardly.When considering this point, it should be remembered that when the heattransport element 204 is operating the bubbles 222 will be formingwithin established density driven circulation fluid flows and will moveupwardly carried by these flows in addition to the bubbles movement dueto their own buoyancy relative to the liquid water. Further, it isbelieved that expansion of the bubbles 222 as they move upwardly willfurther increase the speed of the density driven circulation flow byincreasing the buoyancy of the expanding bubbles 222. In some examplesexpansion of the bubbles as they move upwardly may also increase thedegree of pistonic driving.

This density driven circulation may form a highly effective heattransport mechanism because water has a relatively high enthalpy ofvaporization, so that the movement of the bubbles 222 of water vapor maycarry a large amount of heat energy, in addition to the heat energycarried by the movement of relatively hot water out of the front fluidflow channels 217, and its replacement by cooler water. In arrangementswhere pistonic driving of the flow of the liquid water by the watervapor bubbles takes place the effectiveness of the heat transportmechanism may be further increased by the increase in the flow rate ofthe liquid water caused by the pistonic driving. This pistonic drivingis a component of the overall density driving producing the densitydriven circulation. The pistonic driving is caused by the densitydifference between the liquid water and the bubbles of water vapor.

In general, the speed of the density driven circulation increases andthe effectiveness of the heat transport mechanism increases as thetemperature of the upper sheet 214 of the heat transport element 204increases.

The density driven circulation of the water 221 within the fluid flowchannels 217 and 218 is a vapor driven circulating or rolling flow.

The density driven circulation of the water 221 within the fluid flowchannels 217 and 218 becomes particularly vigorous, and becomesparticularly effective as a heat transport mechanism, when thetemperature of the upper sheet 214 of the heat transport element 204becomes sufficiently high that the water 221 within the fluid flowchannels 217 and 218 enters a rolling boil state. The effectiveness ofthe heat transport mechanism significantly increases when rollingboiling of the water 221 commences. In general, when other parameters ofthe system remain constant, entry into the rolling boil state will takeplace when the temperature of the front sheet 214 of the heat transportelement 204 reaches a specific temperature.

In the illustrated examples using water, the water 221 within fluid flowchannels 217 and 218 may enter a rolling boil state at a temperature ofabout 40° C.

The arrangement of the heat transfer element 204 into sections 204 c to204 e with separate fluid flow channels 217 extending along the heattransport element 104 may allow the vertical height of the liquid waterin each section 204 c to 204 e of the heat transport element 204 to bereduced compared to embodiments in which the density driven flow extendsalong the length of a heat transport element, and so reduce the pressurehead acting on the liquid water at the bottom of the heat transportelement 204. In general, increased pressure reduces the tendency ofliquids to vaporize and so increases the boiling point of liquids.Accordingly, reducing the pressure head acting on the liquid water atthe bottom of the heat transport element 204 may increase the tendencyof the liquid water 221 in the front fluid flow channels 217 to vaporizeand produce bubbles 222, and so may improve the efficiency andeffectiveness of the heat transport element 204.

In particular, the reduction of the pressure head acting on the liquidwater at the bottom of the front fluid flow channels 217 may reduce anytemperature differential along the lengths of the front fluid flowchannels between their the top and bottom ends by reducing anydifference in the tendency of the liquid water to vaporize due todifferences in pressure. This may reduce temperature differentialsbetween the different points on the heat transport element 204 and mayavoid the formation of hot spots in the photovoltaic elements 205.Accordingly, reducing the pressure head acting on the liquid water atthe bottom of the heat transport element 204 may make the temperature ofthe front sheet 214 of the heat transport element 204 more isothermal.

The arrangement of fluid flow channels 217 extending longitudinallyalong the heat transport element 204 and interconnected by vapormanifolds 211 extending laterally across the heat transport element 204may allow a very rapid flow of heat energy along the heat transportelement 204 away from any fluid flow channel 217 having a highertemperature. This may reduce temperature differentials between thedifferent points on the heat transport element 204 and may reduce, oravoid, the formation of hot spots in the photovoltaic elements 205.

The provision of the two separate heat transport mechanisms of themovement of water vapor along the vapor manifold 211 and the densitydriven flow of liquid water and water vapor along each of the frontfluid flow channels 217, respectively acting longitudinally andtransverse the length of the heat transport element 204 may tend toequalize the temperature across the entire upper surface of the heattransport element, and thus tend to equalize the temperature across thephotovoltaic elements 205 and reduce, or avoid, the formation of hotspots.

The movement of water vapor along the vapor manifold 211 provides a veryrapid heat transport mechanism that tends, by the vaporization andcondensation of water, to move heat energy from relatively hot locationsto relatively cold locations. As a result, the movement of water vaporalong the vapor manifold 211 may tend to equalize the temperature of theliquid water surface at different positions across the heat transferelement 204, in addition to transporting heat energy from the heattransport element 204 to the heat exchanger 107 formed by the tube 234.This temperature equalization may have the effect of removing more heatenergy from hotter parts of the heat transport element 204, and sotending to equalize the temperature across the front surface of the heattransport element 204. It is clear that such isothermal cooling willtend to reduce, or avoid, the formation of hot spots, for example in anyphotovoltaic element attached to the front surface of the heat transportelement 204.

Similarly to the first embodiment, the rear sheet 215 of the heattransport element 204 has a plurality of hollow ridges 225 extendingbetween the flat part of the rear surface 204 b and the semi-cylindricalsurface of each outwardly projecting section 210. Each hollow ridge 225has a ‘V’ profile, and the hollow ridges 225 are located spaced apart atregular intervals along the length of each outwardly projecting section.The hollow ridges 225 act as supports for the outwardly projectingsections 210, and also act as drains to return liquid water from thevapor manifolds 211 into the rear fluid flow channels 218 in a similarmanner to the hollow ridges 125 of the first embodiment.

The hollow ridges 225 may extend the range of angles of inclination atwhich the heat transport element 204 can be used, as explained aboveregarding the first embodiment.

Depending upon the geometry of the different parts of the heat transportelement 204 in any specific design, even when the hollow ridges 225 areused there may still be a minimum angle of inclination at which the heattransport element 204 can operate without the retention of liquid waterin the vapor manifolds 211 having adverse effects on operation of theheat transport element 204.

The corrugated profile of the central sheet 216 and the bonding of thecentral sheets 216 to the front sheet 214 and the rear sheet 215increases the strength and rigidity of the heat transport element 204,and may reduce or prevent ballooning for the reasons discussed regardingthe second embodiment. This may make the heat transport element 204 amore rigid structure. This may tend to reduce the amount of flexing ofthe heat transport element 204 in use. This may prevent damage to thephotovoltaic elements 205 by reducing the amount of mechanical stressapplied to the photovoltaic elements 105. This may allow the front,rear, and/or central sheets 214, 215, 216, to be thinner, which mayreduce weight and costs. This may allow the front sheet 214 to bethinner, which may improve the transfer of heat from the photovoltaicelements 205 into the liquid water within the front fluid flow channels217.

The heat transport element 204 is a substantially rigid structure. Thismay minimize changes in the level of the upper surface 232 of the water221 due to flexing of the components of the heat transport element 204,such as the upper and lower sheets 214 and 215. Such changes in thelevel of the upper surface 232 of the water 221 may affect theefficiency of the cooling of the photovoltaic elements 205.

As is explained above, the interior of the heat transport element 204 isevacuated, and the heat transport element 104 is located within anevacuated tube 203. Usually the heat transport element 204 and theevacuated tube 203 are evacuated to the same pressure. In theillustrated example of the second embodiment described above thispressure may be 10⁻³ mbar.

The interconnection of the front and rear sheets 214 and 215 by thelinking surfaces of the central sheet 216 may resist ballooning of thefront and rear sheets 214 and 215 and reduce or prevent ballooning.Arranging for the linking surfaces of the central sheet 216 to bestraight may increase the resistance to ballooning. Reducing orpreventing ballooning may prevent damage to the photovoltaic elements205 by reducing the amount of mechanical stress applied to thephotovoltaic elements 205. This may allow the front sheet 214 to bethinner, which may reduce weight and costs and/or may improve thetransfer of heat from the photovoltaic elements 205 into the liquidwater within the front fluid flow channels 217.

For the same reasons as explained with regard to the first embodiment itis preferred for the sizes of the surfaces of the central sheets 216 incontact with the front sheet 214 to be as small as possible, subject tothe contact area between the central sheets 216 and the upper sheet 214being sufficiently large to form a reliable bond of the requiredstrength.

In the illustrated example of the fifth embodiment 0.2 mm thick tincoated mild steel sheets are used to form the different sheets of theheat transport element. In alternative examples other thicknesses may beused, in particular 0.1 mm thick tin coated mild steel sheets may beused.

In the illustrated example of the fifth embodiment the spacing betweenthe front sheet 214 and the parallel parts of the rear sheet 215 is 1.8mm at the locations of the recesses. Accordingly, the thickness of thefluid flow channels 217 and 218 at the locations of the recesses is 1.6mm, since the thickness of the central sheet is 0.2 mm.

The sheets used to form the heat transport element may be shaped bypressing.

In the illustrated fifth embodiment the heat transport element 204 isarranged to be horizontal transversely to longitudinal axis. That is,the vapor manifolds 211 should be horizontal. However, in practice somedeviation from the horizontal may be tolerated without significantimpact on the operation of the heat transport element 204. Suchdeviation from the horizontal will result in differences in the level ofthe liquid water surface relative to the structure of the heat transportelement 204 at different positions along the length of each vapormanifold 211. As is explained above, the level of the liquid watersurface may be varied. Accordingly, the minor differences in levelcaused by small deviations from the horizontal may be accommodated.

The front and rear sheets 214 and 215 of the fifth embodiment have adimpled profile similarly to the upper and lower metal sheets 114 and115 of the second embodiment.

As discussed above the heat transport element 204 has a flat frontsurface 204 a formed by a front sheet 214 with a dimpled profile. Inaddition, the front sheet 214 is has two longitudinal recesses runningacross in its front surface 204 a which form two parallel troughsrunning along the upper surface 204 a of the heat transport element 204behind the photovoltaic elements 205. Similarly to the first embodimentelectrically conductive ribbons or wires run along the longitudinalrecesses between the heat transport element 204 and the photovoltaicelements 205. The wires are electrically connected to the photovoltaicelements 205 and to the conductors 21 which pass through the cap 12 toprovide a conductive path to carry the electrical power generated by thephotovoltaic elements 205 out of the sealed transparent tube 203. Thiselectrical power may be supplied to an inverter for voltage conversionand/or for conversion to alternating current for supply to a domestic ormains electrical system.

In examples where adhesive is used to attach the photovoltaic elements205 to the heat transport element 204, an electrically insulatingadhesive can be used in a similar manner to the second embodiment.

In the fifth embodiment the longitudinal recesses run parallel to thefluid flow channels 217 and 218. Accordingly, each of the longitudinalrecesses can be accommodated by reducing the thickness of one of thefront fluid flow channels 217 in each section 204 c to 204 e of the heattransfer element 204.

In the illustrated example of the fifth embodiment the spacing betweenthe front sheet 214 and the parallel rear sheet 215 is 1.8 mm at thelocations of the longitudinal recesses 129. Accordingly, the thicknessof the front fluid flow channels 217 at the locations of thelongitudinal recesses is 1.6 mm, since the thickness of the centralsheet is 0.2 mm.

The heat transport element of the fifth embodiment may be formed usingthe same materials and bonding techniques as in the first embodiment.

In the illustrated example of the fifth embodiment the flow of watervapor and liquid water through the heat transport element 204 tends tokeep the cooled front surface of the heat transport element 204 at auniform operating temperature during operation. That is, the cooledupper surface of the heat transport element 104 tends to be keptisothermal. The isothermal nature of the cooled upper surface of theheat transport element 104 tends to give rise to isothermal cooling ofthe photovoltaic elements 105, where hotter parts of the photovoltaicelements 105 tend to be preferentially cooled so that the photovoltaicelements 105 themselves tend to become isothermal

Such isothermal cooling provides further advantages in addition to thoseprovided by cooling.

Isothermal cooling may provide the advantage that the appearance of hotspots or regions in the photovoltaic elements 205 produced by heating byincident solar radiation can be reduced or eliminated. Such hot spots orregions can reduce the efficiency of the photovoltaic elements 205.

Isothermal cooling may simplify the control and wiring arrangements ofthe photovoltaic elements 205 by reducing or eliminating any requirementfor compensation for differences in the performance of the differentparts of the photovoltaic elements 205 that are at differenttemperatures.

Isothermal cooling tends to reduce, or prevent, the formation of hotspots or regions in the photovoltaic elements 205. As is explainedabove, this may allow the efficiency of the photovoltaic elements 205 tobe improved at a specific temperature. Further, this may reduce theamount of degradation of the photovoltaic elements 205 caused by highertemperatures.

Still further, this may allow the photovoltaic elements 205 to operatewith a given degree of efficiency at a higher temperature than wouldotherwise be the case. This may allow the solar energy collectorassembly 202 including the photovoltaic elements 205 to be operated at ahigher temperature without reducing the efficiency with which thephotovoltaic elements 205 produce electrical energy.

One example of this effect of isothermal cooling is that the generalfigure quoted above for silicon photovoltaic elements that theefficiency of electrical energy generation generally drops by about0.35% to 0.5% for each degree centigrade of temperature increase above25° C. may not apply to silicon photovoltaic elements that areisothermally cooled. Such isothermally cooled silicon photovoltaicelements having hotspots eliminated or reduced may have a higherthreshold temperature at which the efficiency of electrical energygeneration begins to drop and/or may have a reduced rate of reduction inefficiency for each degree centigrade of temperature increase above thethreshold temperature. Further, the temperature at which there is a riskof permanent degradation of the silicon photovoltaic elements may alsobe increased for isothermally cooled silicon photovoltaic elements.Similar effects may be found in photovoltaic elements formed of othersemiconductor materials.

In some examples, one or more layers of heat conductive material may belocated between the upper sheet 214 and the photovoltaic elements 205.Such layers of heat conductive material may increase the rate of heattransfer between the photovoltaic elements 205 and the front sheet 214,and thus the rate of heat transfer between the photovoltaic elements 205and the liquid within the front fluid flow channels 217. Such layers ofheat conductive material may also increase the rate of heat transferlaterally across the photovoltaic elements 205.

Accordingly, providing a layer of heat conductive material may increasethe degree of isothermal cooling and further tend to reduce, oreliminate, the formation of hot spots or regions in the photovoltaicelements 205.

The heat transport element may be used in other applications separatelyfrom the rest of the solar energy converter.

In some examples control methods can be used to control the temperatureof the solar energy collector assembly 202. In some examples thetemperature of the solar energy collector assembly 202 may be controlledby changing the rate of removal of heat energy from the solar energycollector assembly 202.

In some examples the rate of removal of heat energy from the solarenergy collector assembly 202 can be controlled by altering the flowrate of the first operating fluid passing through the tube 234 formingthe heat exchanger 207.

In some examples the rate of removal of heat energy from the solarenergy collector assembly 202 can be controlled by altering the vacuumpressure within the tube 203. This may change the rate of convectiveheat loss from the solar energy collector assembly 202 to the tube 203.In general, heat transferred to the tube 203 will be rapidly lost to theoutside environment by convection and/or conduction.

In some examples the rate of removal of heat energy from the solarenergy collector assembly 202 can be controlled by altering the vacuumpressure within sections 204 c to 204 e of the heat transport element204. In general, the tendency of the liquid water within the front fluidflow channel 217 to vaporize and form bubbles of vapor 222 will increaseas the vacuum pressure is reduced, and the tendency of the liquid waterwithin the front fluid flow channel 217 to vaporize and form bubbles ofvapor 222 will decrease as the vacuum pressure is increased. As isexplained above, the density driven circulation of water around thefront and rear fluid flow channels 217 and 218 and the transport of heatenergy along the vapor manifolds 211 and the tubes 219 are both drivenby water vapor. Accordingly, altering the tendency of the liquid waterto vaporize by altering the vacuum pressure may allow the rate ofremoval of heat energy from the solar energy collector assembly 202, andthe rate of removal of heat energy from the photovoltaic elements 205 tobe controlled, and so allow the temperature of the solar energycollector assembly 202 and photovoltaic elements 205 to be controlled.

Further, the temperature at which rolling boiling of the water 221within the front fluid flow channels 217 commences will tend to increaseas the vacuum pressure is increased, and will tend to decrease as thevacuum pressure is decreased. Accordingly, in examples where the vacuumpressure within the heat transport element 204 is altered thetemperature at which the water 221 within the front fluid flow channels217 commences rolling boiling can be changed.

As is explained above, the density driven circulation of water aroundthe front and rear fluid flow channels 217 and 218 becomes particularlyvigorous, and becomes particularly effective as a heat transportmechanism, when the water 221 within the front fluid flow channels 217enters a rolling boil state. Accordingly, altering the temperature atwhich the water 221 within the front fluid flow channels 217 commencesrolling boiling by altering the vacuum pressure may allow the rate ofremoval of heat energy from the solar energy collector assembly 202 andphotovoltaic elements 205 to be controlled, and so allow the temperatureof the solar energy collector assembly 202 and photovoltaic elements 205to be controlled.

In some examples the temperature of the solar energy collector assembly202 may be controlled by changing the amount of solar energy incident onthe solar energy collector assembly 202, and so changing the rate ofabsorption of heat energy by the solar energy collector assembly 202.

In some examples the amount of incident solar energy may be controlledby changing the orientation of the solar energy collector assemblyrelative to the direction of the incident solar energy. This can becarried out using a drive mechanism able to rotate the solar energycollector assembly about one or more axes.

In some examples the amount of incident solar energy may be controlledusing adjustable light intercepting or blocking mechanisms in the pathof the incident solar energy. In some examples variable filters,shutters, stops, or the like may be used. In some examples theseadjustable light intercepting or blocking mechanisms may comprisephysical devices. In some examples these adjustable light interceptingor blocking mechanisms may comprise devices having electronicallycontrolled optical characteristics, such as liquid crystals.

In examples where the temperature of the solar energy collector assemblyand/or the photovoltaic elements are to be controlled, a temperaturesensor and a temperature controller may be provided, together with atemperature control mechanism arranged to carry out one, some, or all,of the methods of controlling temperature described above.

The temperature sensor is arranged to measure the temperature of thesolar energy collector assembly and provide this temperature value tothe temperature controller. The temperature controller can then operatethe temperature control mechanism in a suitable manner to control thetemperature of the solar energy collector assembly to the desired value.

Examples where the temperature of the photovoltaic elements is to becontrolled a temperature sensor arranged to measure the temperature of aphotovoltaic element or elements and provide this temperature value tothe temperature controller may be provided. This may be additional to,or instead of, the temperature sensor arranged to measure thetemperature of the solar energy collector assembly. The temperaturecontroller can then operate the temperature control mechanism in asuitable manner to control the temperature of the photovoltaic elementor elements to the desired value.

In some examples the temperature sensor can be provided on the uppersurface of the solar energy collector assembly. In some examples thetemperature sensor can be formed on the same semiconductor wafer as aphotovoltaic element.

Conveniently, the temperature controller may be a suitably programmedgeneral purpose computer.

In the illustrated fifth embodiment, the heat transport element 204 isdivided into three sections 204 c to 204 e, each of which has a separateheat transfer system comprising a number of front and rear fluid flowchannels 217 and 218, a vapor manifold 211, and a tube 234. Each ofthese separate heat transfer systems operates in a similar manner to thefirst embodiment described above. In other examples the heat transportelement 204 may be divided into a different number of sections, eachhaving a separate heat transfer system.

In the illustrated fifth embodiment the tubes 234 each extend outwardlyfrom the side of the heat transport element 204, then turn through aright angle and extend parallel to the axis of the tube 203 to passthrough the end cap 220 of the tube 203. In other examples, the tubes234 may be arranged differently. In some examples the tubes 234 may beinterconnected for mutual support. This may improve the support providedto the heat transport element 204. The tubes 234 connect the respectiveheat exchangers 207 to the support structure 206, and may be regarded asa heat exchange network.

In the illustrated fifth embodiment the tubes 234 each extend outwardlyfrom the end of a respective vapor manifold 211. In some examples thetubes 234 may extend from a different part of the respective vapormanifolds 211. In some examples the tubes 234 may extend from differentparts of the respective vapor manifolds 211 from one another.

In the illustrated fifth embodiment the different sections 204 c to 204e of the heat transport element 204 are each divided by a wall 231extending between the front and rear sheets 214 and 215 to form a fluidtight seal between the fluid flow channels of the different sections. Inother examples a different sealing structure could be used. In someexamples the front and rear sheets 214 and 215 could be brought intocontact to form the fluid tight seal. In some examples the rear sheet215 could be bent towards the flat front sheet 214 to contact the frontsheet 214 and form the fluid tight seal. In some examples the rear sheet215 may be shaped by pressing.

The illustrated fifth embodiment is a hybrid solar energy convertercomprising photovoltaic elements and arranged to convert incident solarradiation into outputs of both electrical energy and hot water. In otherexamples the photovoltaic elements may be omitted to provide a solarenergy converter arranged to convert incident solar radiation into anoutput of hot water.

The illustrated example of the fifth embodiment has a heat exchangercomprising concentric tubes according to the third embodiment. In otherexamples the fifth embodiment may have a heat exchanger comprising tubesaccording to those of the first, second or fourth embodiments.

In the illustrated embodiments the heat exchanger is formed by simplesmooth tubes. In other examples the heat exchanger tube or tubes couldbe shaped or surface profiled to increase their surface area and/or toencourage and increase contact between the different fluids and thesurfaces. In some examples the surfaces may be convoluted, threadedand/or spiraled. In some examples the tubes may contain flow controllingor modifying structures to increase or improve the contact between thefirst fluid and the walls of the tube or tubes forming the heatexchanger. In some examples the flow controlling or modifying structuremay comprise a helical insert within the tube.

Temperature Control

The embodiments described above may further comprise, in addition to thedescribed heat exchanger, an additional secondary heat exchanger and aheat transfer control valve. The secondary heat exchanger may beconnected to the vapor manifold by a vapor passage or pipe with the heattransfer control valve arranged to selectively allow, or prevent, thetransfer of heat energy from the heat transport element to the secondaryheat exchanger.

The heat transfer control valve is able to selectively allow, orprevent, the transfer or transport of heat energy from the heattransport element to the secondary heat exchanger. Accordingly, thedegree of cooling applied to the photovoltaic elements can be varied.

In some arrangements the heat energy transferred to the secondary heatexchanger is transferred into ambient air and allowed to escape and thesecondary heat exchanger is used, under the selective control of theheat transfer control valve, to release heat energy in order to regulatethe temperature of the solar energy collector assembly.

The trigger temperature of the heat transfer control valve at which thevalve opens may be predetermined. In some examples the triggertemperature may be settable in use, or on installation or manufacture ofthe hybrid solar energy converter. In some examples the triggertemperature may be settable to different values depending on theintended maximum water temperature of the water to be heated. Inparticular, in some examples the trigger temperature may be settable to65° C. when the hybrid solar energy converter is to be used to heatwater for a domestic hot water system and may be settable to 135° C.when the hybrid solar energy converter is to be used to heat water foran industrial hot water system.

In some examples the trigger temperature of the heat transfer controlvalve may be selected to maximize the generation of electrical energy bythe photovoltaic elements. In some examples the trigger temperaturevalue may be selected to increase the amount of heat energy transferredto the first operating fluid. In some examples the trigger temperaturemay be selected to optimize the overall production of energy, takinginto account both the amount of electrical energy produced by thephotovoltaic elements and the amount of heat energy transferred to thefirst operating fluid. In some examples the optimizing may maximize thetotal production of energy. In some examples the optimum overallproduction of energy may take into account the relative demand for, orvalue of, the different types of energy, rather than simply maximizingthe total amount of energy produced.

As explained above, the isothermal cooling tends to reduce, or prevent,the formation of hot spots or regions in the photovoltaic elements. Thismay allow the solar energy collector assembly including the photovoltaicelements to be operated at a higher temperature without reducing theefficiency with which the photovoltaic elements produce electricalenergy. This may allow the temperature of the collector assembly to beincreased to produce more useable heat energy without the increase intemperature reducing the efficiency with which the photovoltaic elementsproduce electrical energy. This may allow the trigger temperature to beincreased.

In some examples the trigger temperature may be set to differenttemperatures during use of the hybrid solar energy converter. This mayallow the temperature of the collector assembly to be controlled toproduce different amounts of useable heat energy or electricitydepending upon which type of energy is most in demand at a specifictime.

For example, when hot water is more in demand than electricity the valvemay be closed to pass hot water vapor from the heat transport elementonly to the heat exchanger to maximize the amount of heat applied to thewater acting as the first operating fluid regardless of any temporaryreduction in efficiency of the photovoltaic elements as a result of anyresulting increase in temperature of the collector assembly. Further,when hot water is less in demand than electricity, the valve may beopened in order to pass hot water vapor from the heat transport elementto both of the heat exchangers in order to cool the photovoltaicelements as much as possible and maximize the efficiency of electricitygeneration regardless of the effects on the temperature of the wateracting as the first operating fluid.

Alternative Collector Arrangements

The illustrated embodiments all employ a single substantially flatcollector assembly within a tube. Other arrangements may be used.

In some examples the collector assembly may be curved. The curvedcollector assembly may be arranged to have a curved outer surfaceconcentric with a cylindrical tube within which the collector assemblyis mounted. This may allow a collector assembly having a greater surfacearea to be fitted within a cylindrical tube of a particular size. Thecurved collector assembly may have curved photovoltaic elements mountedon it.

Some examples may mount multiple collector assemblies within a singletube.

Some examples may mount multiple collector assemblies at differentangles within a single tube. In examples where the collector assembliesand the tube are fixed this may allow the efficiency of the collector tobe increased by arranging the different collector assemblies at anglesadapted to more efficiently collect energy at different times of day.

In some examples mirrors and/or lenses may be associated with the hybridsolar energy converter to direct or focus incident solar energy onto thecollector assembly. Such mirrors may be flat or curved. Such mirrorsand/or lenses may be fixed or moveable. In some examples moveablemirrors or lenses may be arranged to track the sun.

In some examples the transparent tube may incorporate a lens to director focus incident solar energy onto the collector assembly. In someexamples the transparent tube may incorporate a Fresnel lens.

In the illustrated examples the first fluid is water. In other examplesother fluids may be used, which may be liquids, vapors or gasses.

Sun Tracking

The embodiments described above are solar energy converters whichconvert incident solar radiation into useable electrical and/or heatenergy.

In some examples the collector assemblies of the solar energy convertersmay be arranged to change their orientation to follow the apparentmovement of the sun across the sky, or track the sun. This may increasethe amount of solar radiation energy incident on the collectorassemblies, for well-known geometric reasons, and so may increase theamount of useable electrical and/or heat energy produced.

FIG. 18 shows a general view of a sixth embodiment of a solar energyconverter 300 arranged to be able to change orientation to track thesun.

The solar energy converter 300 comprises a sealed transparent tube 301containing a solar energy collector assembly 302 and mounted to a heatexchange assembly 303. The solar energy converter 300 may be a solarenergy converter according to any of the embodiments disclosed herein.Sun tracking arrangements may be added to any of the embodiments.

In the illustrated example of the sixth embodiment the sealedtransparent tube 301 is cylindrical and has an axis 304. The sealedtransparent tube 301 is mounted for rotation about the axis 304 togetherwith the solar energy collector assembly 302 mounted within the tube301. A drive motor 305 is arranged to rotationally drive the tube 301through a transmission mechanism 306. In the illustrated example thetransmission mechanism 306 is a cog and chain transmission mechanism.

By selectively operating the drive motor 305 based on the time and date,the sealed transparent tube 301 and solar energy collector assembly 302can be rotated to follow the sun as the apparent position of the sunchanges as a result of the rotation of the earth.

Adding such a solar tracking drive system may increase the amount ofenergy gathered by the solar energy collector assembly by about 20%.

FIG. 19 shows a general view of a seventh embodiment of an array 307 ofsolar energy converters 300, In FIG. 19 a plurality of solar energyconverters 300 according to the sixth embodiment are mounted to form anarray 307. Each of the solar energy converters 300 comprises a sealedtransparent tube 301 containing a solar energy collector assembly 302and mounted to a heat exchange assembly 303. Each sealed transparenttube 301 is mounted for rotation about an axis 304 together with thesolar energy collector assembly 302 mounted within the tube 301. Thetransparent tubes 302 are mounted on the array 310 so that theirrespective axes of rotation 304 are parallel.

A drive motor 311 is arranged to rotationally drive the tubes 301 of thearray 310 in synchrony through a transmission mechanism 312. In theillustrated example of the seventh embodiment the transmission mechanism312 is a cog and chain transmission mechanism.

The array 310 is mounted on a turntable 313 for rotation about an axis314 perpendicular to the axes 304. A drive motor 315 is arranged torotationally drive the turntable 313 through a transmission mechanism316. In the illustrated example the transmission mechanism 316 is ageared transmission mechanism.

By selectively operating the drive motors 305 and 315 based on the timeand date, the sealed transparent tubes 301 and solar energy collectorassemblies 302 of the array 310 can be rotated to follow the sun as theapparent position of the sun changes as a result of the rotation of theearth.

Adding such a dual axis solar tracking drive system may increase theamount of energy gathered by the solar energy collector assemblies 302by up to about 48%.

In the examples of FIGS. 18 and 19, the operating of the drive motor ormotors should take into account the location of the solar energyconverter or converters 300.

FIG. 20 shows a general view of a eighth embodiment of a solar energyconverter 400 arranged to be able to change orientation to track thesun.

The solar energy converter 400 comprises a sealed transparent tube 401containing a solar energy collector assembly 402 and mounted to a heatexchange assembly 403. The solar energy converter 400 may be a solarenergy converter comprising a heat transport device according to any ofthe first to sixth embodiments described above. Sun trackingarrangements may be added to any of the embodiments.

In the illustrated example of the eighth embodiment the sealedtransparent tube 401 is cylindrical and has an axis 404. The sealedtransparent tube 401 has two opposed open ends sealed by respective endcaps 420. The solar energy collector assembly comprises a heat transportelement 404 according to any of the first to sixth embodiments coveredby photovoltaic elements. The heat transport element 404 is cooled byheat exchanger supplied with a first fluid through tube sections 405,which pass through the end cap 420 at one end of the transparent tube401.

The heat transport element 404 is supported at one end by the tubesections 405 as discussed above. In order to support the other end ofthe heat transport element 404 inward projections 406 are provided on aninner face of the end cap 420. These inward projections 406 are arrangedto allow sliding movement of the heat transport element 404 parallel tothe axis of the transparent tube 401 in order to accommodatedifferential thermal expansion of the heat transport element 404 and thetransparent tube 401.

The sealed transparent tube 401 is mounted for rotation about an axis407 midway between the tube sections 405 together with the solar energycollector assembly 402 mounted within the tube 401.

In the illustrated example the heat transport element 404 is suppliedwith fluid through pipes 405 passing through an end cap 420 at one endonly of the transparent tube 401. In other examples the heat transportelement may be supplied by a single tube passing through an end cap 420at one end only of the transparent tube 401, or by tubes passing throughthe end caps 420 at both ends of the transparent tube 401. In exampleswhere a single tube passes through the end cap at each end of thetransparent tube it is preferred that positions where these tubes passthrough the respective end caps are aligned. In examples where only asingle tube passes through the end cap at one or both ends of thetransparent tube the transparent tube may be mounted for rotation aboutan axis coincident with the tube.

In other examples tube may be mounted for rotation about the axis of thetube.

In other examples the heat transport element 404 may be supported onlyat one end by the tube sections 405.

A solar tracking drive system may be arranged to selectively rotate thesolar energy converter 400 so that the sealed transparent tube 401 andsolar energy collector assembly 402 can be rotated to follow the sun asthe apparent position of the sun changes as a result of the rotation ofthe earth. Carrying out such rotation may increase the amount of energygathered by the solar energy collector assembly by about 20%.

FIG. 21 shows general view of a ninth embodiment of a solar energyconverter array 500 arranged to be able to change orientation to trackthe sun.

In FIG. 21 a plurality of solar energy converters 400 according to theeighth embodiment are mounted to form an array 500. The solar energyconverters 400 are mounted horizontally in a frame 501, and are arrangedfor rotation about their respective axes 407. The solar energyconverters 400 are arranged with their respective axes of rotation 407horizontal and parallel. The array 500 comprises suitable motors anddrive mechanisms located within the frame 501 to carry out rotation ofthe solar energy converters 400, but these are not visible in thefigures.

The array 500 is arranged to synchronously rotate the solar energyconverters 400 about their respective axes 407. In other examples thearray 500 may be arranged to allow separate rotation of the individualsolar energy converters 400.

The frame 501 comprises fluid supply tubes 502 and a fluid supplynetwork allowing the first fluid to be supplied to and from each of theheat transport elements 404 of the solar energy converters 400. In theillustrated example the first fluid is water.

The frame 501 is mounted for rotation about a vertical axis on a bearingsection 503 to allow the array 500 to change orientation to track thesun. The vertical axis of rotation of the frame 501 is perpendicular tothe axes of rotation of the individual solar energy converters 400. Thearray 500 comprises a suitable motor and drive mechanism located withinthe frame 501 to carry out rotation of the array 500, but these are notvisible in the figures.

By selectively rotating the solar energy converters 400 and the array500 based on the time and date, the sealed transparent tubes 401 andsolar energy collector assemblies 402 can be rotated to follow the sunas the apparent position of the sun changes as a result of the rotationof the earth.

Adding such a solar tracking drive system may increase the amount ofenergy gathered by the solar energy collector assembly by about 48%.

In general, it is not necessary to rotate the solar energy converters400 freely, but only through a limited angular range. Accordingly, itmay not be necessary to use fully rotating joints to connect the tubesections 405 to the fluid supply tubes 502. In some examples coiledpipes allowing a limited range of angular movement by coiling oruncoiling as the solar energy converters 400 rotate may be used. Wherethere are two tube sections 405 connected to a solar energy converter400 they may be coiled in opposite senses to reduce any change in thetorsional forces they apply to the solar energy converter 400 as itrotates.

In the illustrated example there are six solar energy converters 400 inthe array 500. In other examples a different number of converters 400may be used.

In the illustrated example the solar energy converters are arrangedhorizontally. In other examples they may be arranged in otherorientations. In one example they may be arranged vertically.

In the illustrated example the solar energy converters are arranged forrotation about respective horizontal axes. In other examples they may bearranged to rotate about axes with other orientations. In one examplethey may be arranged to rotate about respective vertical axes.

In the illustrated example the array is arranged for rotation about avertical axis. In other examples the array may be arranged for rotationabout an axis with a different orientation. In some examples the arraymay be arranged to rotate about an axes perpendicular to the axes ofrotation of the solar energy converters.

In examples where the solar collectors can be rotated about one or twoaxes to follow the sun, rotation about a single axis may increase theamount of energy gathered by up to about 20%, while rotation about twoaxes may increase the amount of energy gathered by up to about 48%.

In some examples the solar energy collector assembly may be mountedwithin the tube for rotation relative to the tube and a drive motorarranged to rotationally drive the solar energy collector assembly only.In such examples a drive mechanism which will not allow air leakage,which would destroy the vacuum within the tube, should be used.

In some examples the solar energy collector assembly, or the solarenergy collector assembly together with the tube, may be rotated aboutan axis other than the axis of the tube.

General

In the description above the level of water within the heat transportelements of the different embodiments is referred to. The references tothe level of water refer to the level of water when the heat transportelement is cold and the liquid water contains essentially no bubbles ofwater vapor. It will be understood from the above description that thelevel of the water will vary during operation of the heat transportelements as water vapor bubbles are formed in the liquid water andburst, and as the liquid water is vaporized and the water vaporcondenses.

In the illustrated embodiments the heat transport elements may have anoperating temperature range from just over 0° C. to about 270° C. Inpractice, the operating temperature range for domestic instillations maybe limited to a maximum temperature of 95° C., or of 65° C., for safety,and to comply with legal requirements in some jurisdictions. Wheresilicon photovoltaic elements are used the optimum temperature range tomaximize the generation of electricity may be in the range 20° C. to 65°C., or in the range 20° C. to 30° C., or in the range 25° C. to 30° C.

The heat transfer rate of the heat exchanger, that is the rate at whichthe heat exchanger can transfer heat energy from the heat transferelement to the first respective operating fluids, may be matched to theheat transfer rate of the heat transfer element, that is the rate atwhich the heat transfer element can transfer heat from the isothermallycooled face of the collector assembly to the heat exchanger assembly, atthe expected operating temperature, or over the expected operatingtemperature range, of the system. This may improve efficiency.

In the illustrated embodiments the first operating fluid is water. Inother examples the primary operating fluid may be air.

In other examples the first operating fluids may be fluids other thanwater and air.

In the illustrated embodiments a transparent tube or envelope is used.In other examples this may be replaced by a translucent or partiallyopaque tube or envelope.

In general, in all of the embodiments it may be preferred to have thephotovoltaic elements as thin as possible to ensure effective cooling ofthe entire thickness of the photovoltaic elements by the heat transportelement. This may assist in preventing localized hot spots of elevatedtemperature developing within the photovoltaic elements, which hot spotsmay degrade the performance and reliability of the photovoltaicelements. However, in practice there may be a minimum required thicknessof the photovoltaic elements for other reasons, for example physicalstrength.

In the illustrated embodiments degassed distilled water is used. Thismay provide the advantage that the tendency to vaporize of the water ismaximized, increasing the efficiency of the heat transfer by the thermosiphon. Impurities dissolved in the water, including dissolved gasses,will tend to suppress vaporization of the water.

In some examples the water may contain vaporization enhancing additivesto increase the tendency of the water to vaporize. In some embodimentsparticles of hydrophobic materials may be used, in particular particlesof zinc oxide may be used. The particles of hydrophobic molecules mayact as nucleating sites, boosting the formation of bubbles of watervapor, without tending to suppress vaporization.

In all of the embodiments, nucleation enhancing structures may be addedto the surfaces of the riser channels only, and not the return channels.This may encourage the liquid water to vaporize and form bubblesprimarily, or only, in the riser channels even when the water in theriser and return channels are at similar, or the same, temperature.Suitable nucleation enhancing structures may include micropores and/orsurface roughening.

In all of the embodiments, pores or apertures may be provided in thesheet separating the riser and return channels to allow water to passfrom the return channel to the riser channel. This may improve thecirculation of the liquid water and improve the efficiency of the heattransfer.

In the illustrated embodiments water is used as the working fluid withinthe heat transport element to provide the density driven circulation. Inother embodiments other vaporizable liquids, solutions or mixtures maybe used. In particular a mixture of water and glycol may be used,ethanol may be used, and a mixture of ethanol and water may be used.Mixtures of dissimilar fluids where one fluid acts as a nucleating agentfor another fluid may be used.

In other examples a mixture of 75% water and 25% ethanol may be used asthe working fluid within the heat transport element. When a mixture of75% water and 25% ethanol is used the mixture may enter a rolling boilstate at a temperature of about 22° C. In other embodiments the relativeproportions of water and ethanol used as the working fluid may be variedin order to set the temperature at which a rolling boil commences to adesired temperature.

As discussed above, the effectiveness of the heat transport mechanismsignificantly increases when rolling boiling of the working fluidcommences. Accordingly, it applications where it is desirable to keepthe temperature of the cooled face of the collector assembly below aspecific temperature, it may be preferred to select a working fluid, ormixture, which commences rolling boiling at a temperature at or belowsaid specific temperature at the intended vacuum pressure conditionswithin the heat transfer device.

In the illustrated embodiments the tubes forming the heat exchanger arelocated inside the vapor manifold so that the entire circumference ofouter surface of the tube is inside. In other examples the tubes may bearranged so that only a part of the tubes project inside the vapormanifold,

In examples where the solar energy collector assembly rotates relativeto the evacuated tube a rotating vacuum seal must be provided betweenthem. In some examples a rotating vacuum seal may be provided by amulti-stage seal. In particular a multi-stage O-ring seal may be used.

Where a multi-stage O-ring seal is used an advantageous method ofmanufacture may be to form the O-ring seals of the different stages inorder from the interior of the evacuated tube to the exterior whileevacuating the tube. This will provide a multi-stage O-ring seal withthe regions between the seals initially having the same vacuum pressureas the interior of the tube. Such a multi-stage O-ring seal may supporta long lasting vacuum within the tube even when the multi-stage O-ringseal is used as a rotating vacuum seal.

The above embodiments illustrate and describe a single solar energyconverter. In practice an array made up of a plurality of such units maybe used. In such an array each solar energy converter may have adedicated electrical inverter. Alternatively, a group of a plurality ofsolar energy converters may share a common inverter.

In an array of solar energy converters it may be preferred to have aprimary operating fluid channel running through the primary heatexchangers of all of the energy converters of the array as a commonmanifold.

In an array of solar energy converters it may be preferred for adjacentsolar energy converters to have their respective inlet opening andoutlet opening connected directly together. This may be done byproviding a flange around each inlet opening and outlet opening andclamping together the flanges of the adjacent inlet opening and outletopening of adjacent solar energy converters.

In an array of solar energy converters it may be desirable to be able toextract individual solar energy converters from the array for servicing,or to replace faulty converters, without having to drain all of thefluid from the common manifold. Accordingly, fluid cut off valves may beprovided in the support element of each solar energy converter in orderto seal the appropriate one of the inlet opening or outlet opening whenan adjacent solar energy converter is removed from the array.

The embodiments described above comprise a collector assembly within anevacuated cylindrical tube. In some examples the collector assembly maybe located within an enclosure which is not evacuated. In some examplesenclosures which are not cylindrical tubes may be used.

The embodiments set out above are described in the context of a hybridsolar energy converter. The different parts of the described hybridsolar energy converter may be useable independently.

In particular, the solar energy collector assembly and the heat exchangeassembly may be used in a flat panel device without a separate evacuatedtransparent tube for the solar energy collector assembly. Such a flatpanel device may be evacuated, or alternatively may not be evacuated.

In particular, the collector assembly may be used as a thermal collectorto gather heat energy from incident solar radiation without anyphotovoltaic elements being mounted on the collector assembly.

An array of solar energy converters may comprise both hybrid solarenergy converters with photovoltaic elements mounted on the collectorassembly and thermal solar energy converters without photovoltaicelements mounted on the collector assembly. Such an array may be used toheat water, with the hybrid solar energy converters heating the water toan intermediate temperature and the thermal solar energy convertersheating the water from the intermediate temperature to a hightemperature. The thermal solar energy converters without photovoltaicelements may operate at a higher temperature than the hybrid solarenergy converters because they do not have any photovoltaic elements tosuffer thermal degradation.

In some examples the collector assembly may be used as a thermalcollector to heat air or water in industrial or domestic applications.In some examples the collector assembly may be used as a thermalcollector to heat water in a desalination or water purifyingapplication.

In particular, the heat exchange assembly may be used separately insolar energy heat collectors without the photovoltaic elements and/orwithout the heat transport element. This may allow the problem ofstagnation to be solved.

In particular, the heat transport element may provide a density drivenheat transport mechanism useable in other heat transport applications.

In particular, the heat transport element may provide an isothermalcooled surface useable in other applications.

In particular, the isothermal cooled surface may be curved. This mayallow curved objects to be cooled more efficiently.

In one example the heat transport element may be used to cool electricalcircuits, for example in a computer.

If the heat transport element is used in other applications, and not inconjunction with photovoltaic elements, the heat transport element mayoperate at a wider range of temperatures. In one example the heattransport element using water as the working fluid may operate at atemperature of up to 280° C. In other examples other fluids may be usedas the working fluid. In one example of a high temperature applicationsodium may be used as the working fluid within the heat transportelement.

In some examples the heat transport element may transport heat to one ormore electro-thermal power generators in place of one or both heatexchangers. This may increase the amount of electrical energy generated.In particular the heat transport element may transport heat to aStirling engine or engines.

In the illustrated embodiment vacuums are used within the heat transportelement having a pressure of about 10⁻³ mbar. Higher or lower pressuresmay be used. In general, it is expected that using lower vacuumpressures would improve the performance of the hybrid solar energyconverter. In some examples a vacuum pressure of 10⁻² bar or lower maybe used. In some examples vacuum pressures of 10⁻⁶ mbar or 10⁻⁸ mbar maybe used.

A vacuum pressure of 10⁻³ mbar is generally the lowest pressure that canbe provided by simple vacuum pumps, so that the use of this vacuumpressure is convenient as the necessary vacuum pumps are readilyavailable. The use of this vacuum pressure may be economicallyadvantageous in commercial scale production of hybrid solar energyconverters because of the cost of providing a lower vacuum pressure. Inother embodiments higher or lower vacuum pressures may be used.

In the illustrated embodiments the hybrid solar energy converter hasroof and/or wall mounting brackets. In other embodiments differentmounting methods and components may be used.

The description above describes three embodiments. All of theembodiments are closely related and alternatives, explanations andadvantages disclosed in relation to one of the embodiments can generallybe applied in an analogous manner to the other embodiments. Inparticular, elements of one embodiment may be used in the otherembodiments, and analogous elements can be exchanged between theembodiments.

The above description uses relative location terms such as upper andlower and front and rear. These are used for clarity to refer to therelative locations of the referenced parts in the illustrated figures,and should not be regarded as limiting regarding the orientation and/orlocation of parts of embodiments of the invention during manufacture orin use.

Those skilled in the art will appreciate that while the foregoing hasdescribed what are considered to be the best mode and, whereappropriate, other modes of performing the invention, the inventionshould not be limited to specific apparatus configurations or methodsteps disclosed in this description of the preferred embodiment. It isunderstood that various modifications may be made therein and that thesubject matter disclosed herein may be implemented in various forms andexamples, and that the teachings may be applied in numerousapplications, only some of which have been described herein. It isintended by the following claims to claim any and all applications,modifications and variations that fall within the true scope of thepresent teachings. Those skilled in the art will recognize that theinvention has a broad range of applications, and that the embodimentsmay take a wide range of modifications without departing from theinventive concept as defined in the appended claims.

1. A heat transfer assembly comprising: an elongate envelope; anelongate heat transfer device located within the envelope, the heattransfer device having an elongate heat transfer chamber; and anelongate heat exchanger passing longitudinally through at least aportion of the elongate heat transfer chamber.
 2. A heat transferassembly according to claim 1, wherein the elongate envelope istransparent or translucent.
 3. A heat transfer assembly according toclaim 2, wherein the elongate envelope is glass.
 4. A heat transferassembly according to claim 1, wherein the elongate heat transferchamber extends along substantially the whole length of the elongateheat transfer device.
 5. A heat transfer assembly according to claim 1,wherein the elongate heat exchanger extends along substantially thewhole length of the elongate heat transfer chamber.
 6. A heat transferassembly according to claim 5, wherein the elongate heat exchangercomprises a tube which passes through a first end of the elongate heattransfer chamber, extends along substantially the whole length of theelongate heat transfer chamber, and turns back to pass for a second timethrough the first end of the elongate heat transfer chamber.
 7. A heattransfer assembly according to claim 6, wherein the elongate heatexchanger comprises a tube which passes twice through a first end of theelongate envelope.
 8. A heat transfer assembly according to claim 5,wherein the elongate heat exchanger comprises a tube which passesthrough a first end of the elongate heat transfer chamber, extends alongsubstantially the whole length of the elongate heat transfer chamber,and passes through a second end of the elongate heat transfer chamberopposite the first end.
 9. A heat transfer assembly according to claim8, wherein the tube and/or the elongate heat transfer device comprisemeans to accommodate differential thermal expansion between the tube andelongate heat transfer device.
 10. A heat transfer assembly according toclaim 9, wherein the means to accommodate differential thermal expansioncomprise a bellows structure of the tube.
 11. A heat transfer assemblyaccording to claim 8, wherein the elongate heat exchanger comprises atube which passes through a first end of the elongate envelope, extendsalong substantially the whole length of the elongate envelope, andpasses through a second end of the elongate envelope opposite the firstend.
 12. A heat transfer assembly according to claim 11, wherein thetube and/or the elongate envelope comprise means to accommodatedifferential thermal expansion between the tube and elongate heattransfer device, and the elongate envelope.
 13. A heat transfer assemblyaccording to claim 12, wherein the means to accommodate differentialthermal expansion comprise bends in the tube.
 14. A heat transferassembly according to claim 8, wherein the elongate heat exchangercomprises a tube which passes through a first end of the elongateenvelope, passes through the elongate heat transfer chamber, and turnsback to pass for a second time through the first end of the elongateenvelope.
 15. A heat transfer assembly according to claim 5, wherein theelongate heat exchanger comprises inner and outer concentric tubes whichpass through a first end of the elongate heat transfer chamber andextend along substantially the whole length of the elongate heattransfer chamber, wherein the outer concentric tube is closed at an endremote from the first end of the elongate heat transfer chamber.
 16. Aheat transfer assembly according to claim 1, wherein the elongateenvelope is at least partially evacuated.
 17. A heat transfer assemblyaccording to claim 1, and further comprising at least one photovoltaicelement mounted on the elongate heat transfer device.
 18. A heattransfer assembly according to claim 1, wherein the elongate heattransfer chamber is a vapor chamber.
 19. A heat transfer assemblyaccording to claim 18, wherein the vapor chamber is at least partiallyevacuated.
 20. A heat transfer assembly according to claim 1, whereinthe heat transfer assembly is arranged for rotation about a rotationaxis.
 21. A heat transfer assembly according to claim 20, wherein theelongate envelope is cylindrical and has an axis of symmetry, and theaxis of rotation is parallel to the axis of symmetry.
 22. A heattransfer assembly according to claim 21, wherein the axis of rotationand the axis of symmetry are coaxial.
 23. A heat transfer assemblyaccording to claim 7, wherein the heat transfer assembly is arranged forrotation about a rotation axis, and the axis of rotation coincides withthe location of a tube passing through an end of the elongate envelope.24. A heat transfer assembly according to claim 7, wherein the heattransfer assembly is arranged for rotation about a rotation axis, andthe tube passes twice through the first end of the elongate envelope andthe axis of rotation passes between the locations at which the tubespassing through the end of the elongate envelope.
 25. A heat transferassembly according to claim 24, wherein the axis of rotation passescentrally between the locations at which the tubes passing through theend of the elongate envelope.
 26. A solar collector array comprising aplurality of heat transfer assemblies according to claim 20 mounted inparallel on a common supporting structure.
 27. A solar collector arrayaccording to claim 26, and further comprising means for synchronouslyrotating all of the plurality of heat transfer assemblies relative tothe supporting structure about their respective axes of rotation.
 28. Asolar collector array according to claim 26, and further comprisingmeans for rotating the supporting structure about an axis perpendicularto the axes of rotation of the heat transfer assemblies. 29-98.(canceled)